A conversation I got into with some of my friends turned into specific character design thoughts, and something I think is a big rule is:
Anything you put in your character’s design is a prop for them to express themselves.
Characters need to tell the audience about themselves, and I think one of the best ways to do that is often using habits, tics, body languages, just what they do with themselves. Smiling when happy is a very generalized and universal sort of trait, but if a character has, say, a distinctive overbite and tends to smile crookedly, or only ever smiles the tiniest amount so if they weren’t wearing very striking lipstick you would never catch those movements- that becomes far more distinctive. Those sort of descriptions actively conjure an impression of a character.
Clothing is a prop. Accessories are props. The things your character wears and surrounds themselves with will speak volumes about their routine, their life, their interests, their priorities.
And they’re fantastic for exaggerating or calling attention to character quirks. Have a character who fidgets with things? Give them a lot of necklaces or bracelets or a favorite ring. Character who makes a lot of gestures with their hands? Think of gloves, rings, nail polish- whatever makes sense that will draw the eye to that part of their body.
You can get even more fun giving characters contradictory traits they’re navigating with varying levels of grace- for example, someone with fancy painted nails that have rough, bitten edges because they’re trying to stop chewing them but aren’t quite succeeding yet, or a fashion-conscious type who loves heels but is also a track-and-field star so all their shoes have low sides they can easily kick off and hit the ground running.
If you have a character that shoves their hands in their pockets often, give them noteworthy pockets. Maybe they keep things in there? Kinda hard to get your sulk on when you put your hands on that granola bar you forgot you took with you so now you have to sulk around your newly-discovered snack.