:o well i guess it in part depends on the fic/au. so far i take one aspect (or more) of their persona that i have noticed and is relevant to what im writing and then i emphasize that aspect, if that makes sense?? umm here's a breakdown of my impression based on their public personas^^:
taro- he's obv very very cute and sweet but being the eldest and de facto leader (and having a lot of real world experience like having worked pt and as a dancer, been a student) i think he also could be "strict". i think he's confident esp when it comes to his expertise (ie dancing) and while that can put some people off, i also think it's gained him respect frm others. also i love his dancing style, he makes even difficult moves/choreo look 1. natural 2. fun and effortless and thats something i want to translate to his personality in writing too
es- i think he has a lot of interiority, being generally calm and having a good poker face means he can keep a lot under the surface...which can lead to people projecting a lot of their assumptions onto him. i also think its fun to focus on his more absurd style of humor and his being a weeb/otaku (i say this as a former weeb/anime+manga otaku lol) can lead to funny moments
sc- he's lowkey an ~enigma~ to me haha i think i didnt notice a lot of the depth of his persona until recently (my bad) but i do recall es once writing abt his mood swings/intensity and that had me go "hmm....". he can be a goofy goober but having trained for so long+having seniority/experience as an ex nct member means he also has this maturity and ease...he rly shines in variety content (the zip daesung one had me ctfu) or the yt show he did :D
wb- well ok so obviously as the center/being so insanely good-looking there's a lot of projection onto him, but wrt to him i like to focus on his professional 'quirks' (ie perfectionism) and his neuroticisms (ie the compulsions he mentions like theyre just silly habits), though i rly like how he's become more comfortable being silly in content/variety stuff instead of maybe feeling awkward and at a loss
sh- hmm i feel like the way i see him and the way maybe the fandom sees him dont super align at times...but i usually want to think abt things like his being a neat freak or how he's sensitive (no piercings, doesn't like strong smells like cologne, picky eater) but being such a skilled vocalist i think he's quite confident in that regard (though if i sounded like him i would be singing like 1000x more often in content TT even if it was annoying but im an annoying person so idk) and his relationship w his gender/presentation as a 'small' dude i think could be interesting to explore more idk idk idk i could talk abt him forever
atn- i think i like to focus on w this guy is his subdued persona/sensitive nature and how he's so comfortable in his masculinity that he is also very comfortable w the more 'feminine' aspects of his personality/persona. but also he can be bratty/sassy and thats endearing TO ME.....

uhhh long long answer that probably didnt answer ur question adequately but hey! i tried ^^;;;;;;