There are so many different variations, definitions, and manifestations of chemistry in my opinion, and also contains the continuum of non-romantic to romantic. It's also something of an intangible, which makes me feel very fussy and precise by trying to explain my definitions 😂 However, one of these metrics for me, is "two actors/characters who delight in each other's presence." I've been seeing this in two ongoing dramas with the second lead couple in My Dearest Nemesis and with the main lead couple in Undercover High School. Both couples also demonstrate high "immediacy" with lots of eye contact, smiling, closeness, and small touches. I also think about it as a kind of magnetism, where two people are drawn together - in attention, closeness, and focus - in every scene. One of my favorite examples of this is the main lead couple in Bossam - Steal the Fate. They are drawn to each other in every sense and in my opinion had some intense chemistry. Essentially, in my super scientific explanation, a couple has to have some ✨️sparkle✨️, enjoy each other, and be drawn to each other, and BOTH parties have to be participating in this.

to me its the "unspoken", that intangible push-pull between the leads that makes you want to keep watching even when the other elements of the drama are not that great. (its a bonus if the drama as a whole is well made)

Boyfriend/Encounter
Dali and the Cocky Prince
Crash Landing on you
Its Okay thats Love
Mr Sunshine [cries]
Vincenzo
The Devil Judge

That spark. Or...that sentence. It captures everything so aptly, yet says it all.

Favorite ones:
My Liberation Notes: Mi-jung's "worship me" to Gu Ja-kyung.
Her Private Life: Park Min-young had crazy chemistry with Kim Jae-wook. He matched her aura.
You Are My Spring: Kim Dong-wook and Seo Hyun-jin.
I'll leave it at these because they are exceptional.

I honestly think the perception of chemistry is subjective.
For instance, all my friends (drama addicts too) think that the main couple in Hometown Cha Cha Cha didn't have chemistry at all.
But I didn't notice that supposed lack of chemistry and in fact I have read several times in other forums that Kim Seon Ho and Shin Min A had very good chemistry.

These days I'm finishing Suspicious Partner and I think the main couple don't have chemistry (or the script fails in the romantic sub-plot), but I have also read sometimes that they undoubtly had chemistry.

Another friend of mine usually watch BLs dramas says that the gay couples have much more chemistry than straight couples in regular shows.

For these reasons I think it's subjective.

Chemistry is defined as the branch of science that deals with the identification of the substances of which matter is composed; the investigation of their properties and the ways in which they interact, combine, and change; and the use of these processes to form new substances. In a drama sense, it is defined as the complex emotional or psychological interaction between two people.

What are the things that matter to them?
How do they learn about each other?
How do they interact with each other?
How do their interactions change each other?
What new things grow from their time together?
Does their physical, mental or emotional state change for the better?
And do they understand the change or growth by being together?

I think it's quite a personal feeling.

But for me, it's when the couple feels natural. Even if the writing hits tropes after tropes, if the couple can act them in a natural way and if I don't see the acting, I see chemistry.

Sometimes, I think some actors can have chemistry with anybody, it's like some actors have natural screen presence. It's something you can't really force.

Fun question! I agree with the unspoken thing. That scene in King the Land after the chicken rap when he just stares into her eyes for a couple minutes is so electric and funny.

I feel that with Just Between Lovers, too, so maybe it's Junho.

I think humor is a huge part of it, too. In Call it Love, the obvious chemistry is, well, obvious. But I love the fact that he laughs at her jokes, even when she's not talking to him. Like when they're in the elevator and she's talking to someone else or when he's reading her application for the job. Getting someone's sense of humor is so important. I like when I feel like this couple will have a lot of fun getting to know each other after the story ends.

Also some non-romantic chemistry can be powerful, like in My Mister or Hisaeng (Geu Rae and Mr. Oh are killing me right now).

Chemistry:
There are 8 billion people on earth. Chemistry is when you can’t imagine ANY of those 8 billion people being a better partner than the couple in question. Irreplaceable

For me, chemistry is a sense of connection with one another. In Korea, they use the word "tension" to mean energy. In terms of chemistry, it's the tension caused by the connection of two people, usually fueled by strong emotions, whether that be love or hate. When you see chemistry, you know, and when there isn't chemistry, you sadly know it too.
Best examples of chemistry
Coffee Prince: Gong Yoo and Yoon Eun Hye were so natural in their interactions, whether it was hanging out, fighting, or being affection.
My Liberation Notes; Mr. Gu and Mi Jung's understated but powerful connection was evident even if presented with very few words.
Hyena: Not quite a couple, but the chemistry was palpable.

Non Kdrama Examples
Will Love In Spring: Zhou Yutong and Li Sian
Before We Get Married: Puff Guo and Jasper Wu were on fire with chemistry

Strangely enough, I notice chemistry the most when it *isn't* there. When there is no I-don't-know-what, no je ne sais quoi.
Usually, when there is a clear absence of chemistry, the whole story falls apart (When the Stars Gossip).
On the other hand, when there is a lot of chemistry, the story can still fall through, nevertheless (My Demon, Destined with You).
When a drama has great chemistry, I can occasionally forgive the ending: Flower of Evil.
When the drama hits me with the double amnesia, all the chemistry is undone, My Dearest.
So chemistry is something different for each of us, it's the glue that keeps the drama together and the just-one-more-minute that keeps you watching for me.

Chemistry, if only it was easy to define, actors could simply turn it on and off how they pleased. Sometimes the majority of the audience can agree who has it and who has not.

A series mentioned above was Suspicious Partner. A couple I absolutely adored and still there are others that don't agree.

I really have no definition. It comes down to people I want to see interacting no matter if it is romantic chemistry, friendship or enemies. I would say it's the perfect combination of writing and acting.

I have several thoughts on chemistry. First, let me be clear I am talking about romantic chemistry. There are a lot of friendships portrayed in kdramas that have great chemistry. (I don’t know, because I have never acted, but I feel it’s probably a lot easier to portray friendship ,especially same-sex friendship, than it is to portray a deep love for someone you don’t really care that much about.

But as far as romantic chemistry: I agree that it is pretty subjective on the part of the viewer. I tend to give the main couple the benefit of the doubt, and accept the fantasy/ illusion of their love, so I am always slightly surprised when people criticize a romance mainly on the lack of chemistry. For me chemistry is often as dependent on the writing as the mode of actors’ interaction. A recent example for me was the last episode of Dearest Nemesis, where the writing made me doubt how much the main characters loved each other. That had, for me, nothing to do with the chemistry between the actors but more with the circumstances in which the characters were placed, and the way the script had the characters act.

There are 2 other things I have to say on romantic chemistry that might be different from what others here feel-

1. For me there are certain actors who are excellent actors but who are completely unconvincing when it comes to portraying romantic love. Lee Min-ki and Kim Dong Wook are two male actors like this for me, and one actress, who I list because I’ve often seen her in romances: Shin Hye-sun. I single her out because I feel she is an accomplished actress overall—but I have never said after watching one of her shows “she had such romantic chemistry with her male lead.”

2. Romantic chemistry to me at a certain point requires a physical interaction between the characters—because in my experience, what distinguishes romantic chemistry from friendship chemistry is physical attraction. I am definitely not one of those that says “just that little hand touch showed their chemistry,” especially after they’ve confessed. I realize in real life everyone has a different relationship style, and there are some who are romantically content with occasionally grazing their partner’s pinky finger. Not me. So in terms of fantasy romance, after they have declared their love and are alone together I need to see the couple not JUST having meaningful conversations, but actually kissing, and then when their relationship develops, I want to see them engaging in regular pecks on the cheek and quick hugs in a natural way that shows they are comfortable in and enjoy each others physical presence. If their characters are supposed to be in love, and they are always standing somewhat apart from each other even in a happy ending romance, (and I could list several recent romances where this was the case) I'm never going to think "wow, those actors have a lot of chemistry together."

I completely understand though,...

Hmm, everyone has mentioned a lot of OTPs so I'll focus on the secondary couples and friendships.

First off, chemistry for me is the palpable, subtle tension between two people that makes you desire their kind of relationship in your life.

So examples of familiar chemistry:
I Hear Your Voice - the FL and her mom.
Mother - the lead actress and her daughter.
Doctor Slump - the ML and the FL's mom.
Twinkling Watermelon - Eun Gyeol and his dad (as a teenager 😂😂😂 Does it count?)
Boys over Flowers/Eternal Monarch - the ML and the Housekeeper Ahjumma.

Examples of friendships:
Goblin - Kim Shin and Wang Yeo
Age of Youth - The girls of Belle Epoque
Love in Contract - FL and her best friend
Business Proposal - the FL and SFL
No Gain, No Love/Hometown Cha Cha Cha - the FL and her friend

Second Lead Couples:
Doom at Your Service - Cha Joo Ik and Writer Na
Love All Play - Yuk Junghwan and Lee Yu Min (these two are so cute 🥰)
Love Next Door - Ojirab Queen and Mudflat Man 😂😂😂
Boys over Flowers - Seo Yi Jung and Chu Gaeul (I'm still reading soeulmates fanfics till yesterday 😂😂😂)

Honorable Mentions
Dear M - Jaehyun's character and his best friends 😍😍😍
Serendipity's Embrace - the Bar owner and FL's best friend 😭😭😭

Now that I think about it my favourite dramas turns out to be the ones with intense chemistry (for most cases). Here is my top 5:

1. Coffee Prince (of course, take a look at my handle!) - Nothing beats Eun Chan and Han Gyul <3

2. The Smile has Left your Eyes - The plot has more holes than my grandmas crocheted throw blanket but the leads were amazing together.

3. Jealousy Incarnate - I just love him. Love him and love him and GHJ was amazing with him.

4. Weightlifting fairy - cutest chemistry EVER!

5. Business proposal - second leads!!! Ah!

BONUS:

6. Bossam - not a single on screen kiss and YET the chemistry was intense!

Currently, Seo Kang-joon and Jin Ki-joo in Undercover High School.

To me, the couple has to be visually compatible (read: beautiful) and close in age. The only exception I can think of is Lee Jae-wook and Lee Da-hee in Search: WWW, who is still the best chemistry he has had to date.

Chemistry is completely subjective, but I think writing is the most important factor. Acting is secondary. Actors can't sell any chemistry if the writing gives you second-hand embarrassment, while some talentless actors have a lot of chemistry because of the way their characters are written.

Some of my favorite examples are Mr. Plankton Woo Do-hwan and Lee Yoo-mi and Vincenzo Song Joong-ki and Jeon Yeo-been.

Chemistry is a lot, and there's a kind of chemistry, as some say, that goes for any couple that do not seem stilted and forced when together.
And also for the whole list of characters, that they don't get shy, irritated and/or stiff when they are together, - unless that's in the manuscript.
Personally, I think I am pretty generous seeing chemistry like that - at least I have seen people cry about lack of chemistry where I was ... "Whut?"

But when I hear the word "Chemistry" I more than anything think of the seconds before a kiss (or the seconds of silence where you think "this will be a kiss").
When that works well, I love it, and maybe that's why I am so in awe with the "In one minute, I am gonna kiss you" from "Semantic Error".
https://imgur.com/OTXZgp2
But I said elsewhere on this page, as for that moment, I am a big fan of Seo In Guk.
And as for The Potato Woman, she almost made me cry today, because when a relationship is very unbalanced, that attraction hurts, and that was what I saw. But I'll be speaking about that under the recap.
Oh, and in "Reply 1997" there was a strong demonstration of anti-chemistry which was well played by the actors; a feeling that this was all wrong; https://imgur.com/d1heQBo

One thing I can say from casting a ton of shows is that chemistry often pops up in unexpected places and frequently has little relation to talent or even whether or not the actors actually like it each other in real life. And while the actual sexual orientation of the actors is sometimes a factor in romantic pairings, this isn't always the case - I've had LGBTQ+ actors be completely convincing in straight roles and straight actors be completely convincing in queer roles. Also some actors have the magical ability to generate chemistry to anyone and anything and others just . . . don't. Often you can't tell which pairings will "click" until you get two folks in a room together (or in front of camera) and just see what happens.

It's something that happens when stars align, or you make it align😅. Personally the best example for me is Crash Landing on You. Hyun Bin and Son Ye-jin worked together in The Negotiation before, viewers could see they will be great in romantic project. Then years later Crash Landing on You happen with its big budget to ensure the high quality of the set, outfit, camera works etc. And then yes it was very successful (on and off screen).

But for something like K-2, however much the production wanted Yoon-ah and Ji Chang-wook to have something, it just didn't happen😂. There's another Yoon-ah with a much better chemistry with Ji Chang-wook.

As far as I know K-dramas' lead are offered the scripts. Do they audition or screen/chemistry test? I'm not sure the industry have casting directors.

This is such a good topic that I don't know how to explain. Usually I don't particularly see or feel much chemistry in romantic pairs but I see it during friendships; mainly male friendships but sometimes female friendships.

Like one of the appeals of Weak Hero Class 1 was the rapport between Park Jihoon and Choi Hyun Wook. CHW also had a rapport with Ryeoun in Twinking Watermelon that made that close relationship feel believable (to me at least).
I think Choi Hyun Wook and Lee Seung Gi have better dynamics with their male costars than their female costars; not that it's bad with the ladies, it's just less (to me)

On the ladies' side, all I got is the Business Proposal friendship. Kim Sejeong and Seol Inah seemed like they may genuinely be friends. There just seemed to be a comfort there.

But then I also liked the chemistry from the Family By Choice cast. All 5 main actors had good chemistry to me even though there are age gaps (I think).

I don't know how to put it into words.

When I can feel the cast of a drama feeding off of each other, the gut feeling they give me that they just ‘click’, are comfortable with each other, trust each other, as if they’ve known each other for a very long time (even though irl they may have only just met whilst working on the drama together), and whenever they’re on screen it doesn’t feel like acting to me but an off-script ad-lib or a lived-in everyday natural interaction between them

This is such an interesting question that I pondered a bit on while walking the dogs about which dramas work despite there being no palpable chemistry between the leads.
Goblin very much works, because the chemistry between everyone else is so great - the bromance is off the charts, the characters are fun and surprising and everything just jells, so much that I can sort of very generously overlook / fast forward (okay, not really, because it's still icky) the central sugar daddy love story between a 40 year old looking 400 year old guy and a teenage girl. In some way, even this works, because it adds to the taboo/ forbidden part of the story and the fact that God in this story is a teenage jerk effing up peoples' lifes for shits and giggles.
Another example would be "A Korean Oddysee", also little to no chemistry between the OTP, but the ensemble and the world building is so, so good.

Ah! I was busy all day yesterday and missed this! Hopefully people will still drop by to chat with me farther down the comments.

Chemistry always seems so hard to define, and certainly people disagree on which pairs have chemistry or lack it. What I was getting at the first time I asked this question was really "how does chemistry happen?" because surely it's something that pairs of actors are going for, but not always getting quite right. For me I think chemistry is the result of a lot of elements coming together at the same time, but primarily acting and writing.

The actors have to be bringing their best. If I get taken out of the scene by a weird expression or lack of commitment, there isn't much hope that I'm going to believe their magnetic pull toward someone else. When they are tense, I want it to make me forget to breathe too. But even the best actors can't save a terrible line of dialogue, so the writing also has to be really solid for the genre. Sometimes I wonder how much is getting lost in the translations, but I know I'm picking up more than I was a few years ago! For me, soundtrack can also be a factor, but it's more enhancing or distracting than it is making or breaking the chemistry.

I think chemistry is certainly easier to achieve when both actors are contributing equally, but I don't think it's absolutely a requirement. For example, I'm pretty sure Seo In-guk could have chemistry with a discarded soju bottle if he wanted, but I've definitely seen his efforts not be returned in the same way.

Directors Directors Directors
Directors shape performances
Directors shape how you see what the writer has put in the script
Actors may create a special sparkle, but if the camera angle didn't capture it, if the editing missed its pacing, if if if... then fizzle
And conversely… a director can create tension where in real time there was none... I often think of the romantic moments in Trey Parker's puppet movie Team America when the camera lingers a long time on a pretty face.

Oh, I can't believe I didn't see this topic yesterday! I think about this all the time, thanks for posting the question. To me, the elusive "chemistry" is acting. Why do some actors fail to exhibit "chemistry" when they are showing a romantic (or even, another type!) of relationship? This is a matter of being able to be friendly coworkers in a job that demands emotional vulnerability. There's one thing that has to happen behind the scenes: can the actors be friendly and mutually supportive and not make each other feel too silly getting into character? The other has to happen between the individual actor and the audience: can the actor show the emotions of a fictional character so that the audience believes them?

Recently I watched Agency. I saw others complaining that the one romantic pairing in the show did not have chemistry. I thought their initial romantic scene together was extremely effective. Their knees touched when they sat facing one another, and the bold, silly chaebol heiress had to turn her face away! I thought this an example of a well-directed scene.

One of my favorite shows was You are My Spring. The main pairing were so good together...until their relationship went romantic. Then I couldn't believe in them! I don't know what the deal is there. I suspect strongly that the problem is Kim Dong Wook. I watched a few of his kiss scenes excerpted on Youtube with different co-stars. He is great at the conversation, his voice is so warm, he seems affectionate, he inhabits the character, but when he has to do the kiss it feels like he has fulfilled a contractual obligation. I feel like some actors need an intimacy coordinator for kissing.

(Ooh, did you like the show Love to Hate You? Yoo Teo is a great screen kisser, he's made me believe he was in love with both ladies and gentlemen. On this show he plays an actor who gets super uncomfortable during screen kisses! A fascinating problem.)