so i'm not totally sure if you mean distinguishing characters within a fic or between fics so i'll try to cover both!

within fics first...
for me, i think it's really important that we think about diction. i know it's not super applicable when you have people writing in english about korean characters, but more important than the exact words they use and turns of phrase, is sort of making sure not all your characters sound the exact same or speak like you? i try really hard to think about voice when i can, because i also realize when you speak in real life the kind of things you trip up over and how you structure sentences is quite different than typing it out. for one, i think we speak more in short bursts? it's rare that you'd have someone narrating for paragraphs (sure it does happen tho) so i try not to let one person go on for too long without at least interspersing some action or interruptions. that being said, some people are more wordy than others! i think characters we think of as chatty should get more dialogue lines, and those who are quieter shouldn't be speaking as much or as often. for me personally when i write about jun as a foreigner i also try to give him simpler sentence structure and vocab (in particular, for the soonhui time travel fic i wrote, i'm not proud of the end product, but i did try to pay attention to the fact that jun was a foreign exchange student in the story, so he fumbles for words, he doesn't use complex grammatical organization etc). on the flip side, generally when i write wonwoo i try to give him some words that aren't necessarily common (if it makes sense to do so). and then there are sort of just spoken turns of phrase, for example, in chinese jun says "jiu shi" a lot, which might be figuratively best translated as when we use "like" for a pause? so i think it would make sense to give him hesitations, pauses, markers for things like that. or vernon uses the korean phrase "that kind of feeling" a lot when describing things so that's something else i try to pick up on and incorporate in possible. of course, a reader might not notice any of these distinctions at all! i think that's fine...but when i read i notice when everyone sounds the same or when some speaks in a way i can't imagine them speaking in real life...conversely i also notice when someone's voice is written really "in character" (for me anyway) and it's something i enjoy - off the top of my head there's this one marvel writer who writes a great tony voice i can HEAR when i read his dialogue and i think that's great.
but maybe diction is sort of simpler than setting up the characters themselves...
for me i feel really invested in creating full, complex, rich characters, even if the story itself is short, in my head i like knowing their background, their life story, things like that. and i really try to empathize with each character so for me when i write i often say i don't feel like i'm controlling the character's story or what's happening to them, it feels more like the character is telling me what they would do? for me that takes really getting to know a character (within the story, not necessarily irl) and then that will guide the plot of the story - my initial plans for them might be completely thrown out based on the characters own decisions...they take on a life of their own. and because of that, i think different characters within a story and even between stories will respond to their situations very differently. and i think because of that, conflicts will sort of arise on their own, whether between characters or between a character and their situation or surroundings. so we all sort of navigate our own personal and relationship and work and school challenges simultaneously, but each individual has their own obstacles to overcome that will affect how they interact with each other as well.
it might depend on what narrative voice you use! i prefer a very limited third person voice, but in doing so, i end up stuck in the head of my main character very often? so i need to make sure i step back as well and think about things from other characters' points of view - and they might respond in a totally unexpected way based on the background that i've set up for them, and it might be very unexpected for the main character as well. so i think ultimately it really is developing as much of a backstory as possible (and also future story! like where does that person want to end up (they don't have to end up their, but everyone has something they're moving toward, so i have to know that even if they go off course) and then step into that character's shoes.
for me i think i do this kind of intuitively now, i don't usually write very long character sketches? but again, i think it's really important you know the character's history...
as tips, some of the things you might ask yourself are - what are the characters strengths and weaknesses?  and don't feel like you should make them up entirely or force something on a character...i recently read an explanation about writing weaknesses where a lot of our weaknesses can sort of be too much of a strength? so a character whose good trait might be confidence might also end up being arrogant? not necessarily, but the point is more like, it wouldn't make sense for a character that's super insecure and shy and has low self-esteem and low confidence to be really arrogant, right? the other thing is they don't have to be so obvious or explicit...and sometimes, you know, the strengths and weaknesses only really appear to me as i'm writing the story and i notice a character keeps doing one thing to their detriment, and only then might i see it as a fault...
what motivates your character?
what are their dreams/goals?
what are their fears?
keep in mind for all of these questions, there might be an explicit answer the character is aware of and there might be something that is actually driving the character without them realizing it, but you as a writer might know and that undercurrent can really shape a character and differentiate them from each other.
another thing you might ask yourself is what kind of transformation do you hope to see the character undergo? where are they at the beginning of the fic, and what kind of growth or change do you want to see by the end? i think it's satisfying to see a change or growth, even if it's very very small. but through this question you can also figure out for example the character's attitude (are they optimistic or pessimistic?), their flaws, what's holding them back from what they want, and ultimately their story.
i think if you can think about these sorts of things for each character tho, you can create very different people that interact with each other in a fic, and might end up loving or hating each other...hehe

as for between fics...
i think here, again you need a strong sense of each individual character, but more importantly you need a strong sense of setting. setting will dictate background, or you might think of your own background for the characters first and they end up thrown into a specific setting. how do they react to that? the setting might be a location, a vocation, a time, even just being forced into proximity with another character...but being aware of the situation that the character is about to face, is facing, and also what they have faced and what they will face will really give you their reaction.
i ascribe to this theory in personality where people do have nuances to their behaviours, thoughts, and attitudes, but also humans in general tend to have similar responses to different situations. so i think giving your characters wildly different situations between fics will give different dynamics, and then the real job there is to discover the specific details to their personal responses, how they would differ from someone else.
so, let's take something like waiting in line as an example. a lot of people become impatient, but based on a character's personality, they might take that out on themselves, the person they're waiting for, or someone at home. they might realize they're becoming impatient and try to calm down, or they might not care and relish in their anger, or they might not realize and lack that self awareness. and then when you combine that with the background that you've given the character, i think you can find that by throwing your otp in different settings creates a different way they get together or don't get together as the case may be.
i also, personally, do feel like some otps lend themselves to one specific dynamic more than others. for me, writing wonhui is so interesting because i think i have a very comfortable personal handle on their personalities - whether or not it's true to character i can't tell, but for me myself i can really strongly empathize with either wonwoo or jun in any situation i throw them in and from there develop each of their characters, think about how they might respond to specific situations and to each other, and how they might develop romantically or otherwise or how they might fall apart. that is specifically why i find writing wonhui so fun, because i think i find a different dynamic for them every time i open up a word document. in some universes they fight more, in some one of them likes the other more, in some one of them hates the other or they both hate each other, in some they're much more understanding of each other...but it depends on what their backstories within that fic are, and on what the parameters of the fic are. the way they meet is usually different in each fic, and as a result how they play off each other initially and how their relationship develops is also very different. i have fics where wonwoo starts off distrustful of jun, and fics where jun starts off distrustful of wonwoo, etc. etc. and it's based off situationally how they meet, what's going on in each of their lives when they meet, so their backstories, the setting.

i really don't know if this is helpful at all...i've done workshops where we spent a lot of time on developing character, but i think the last time for FIC writing where i seriously wrote character sketches was like for kismet or kiss me...i will also say that one of the reasons i write so much wonhui is because i think there are so many possible avenues of their relationship to explore? so for each fic i sort of focus on one thing and i try to delve deeply into that... or even if not their relationship then some aspect of their characters... i think variations on a theme is the best example for that - i was really trying to examine the way jun deflects from himself all the time and the way wonwoo introspects... the jeon zone was very much a study on how both of them seem to suck at serious communication lol...

anyway...yeah...this was a lot of writing about nothing i feel like ;______; i really hope there was at least SOMETHING useful in all of it...have a good day anon...again sorry for taking so long