Typically, since short stories are...short stories, sometimes characters aren't fleshed out enough. Sometimes we, as writers, have only a limited amount of time/words to talk/describe our character(s).

So for this week's discussion, when you write stories (preferably short stories), how do you flesh out your character(s). For me, personally, I don't like typing out a character's backstory if it's a short story. I tend to put personality in their dialogue and speech style, and if I have to, I'll give a short description on the character(s).

I say that usually works.
With characters, especially if the main theme is development, is to manipulate their diction choice or actions. A character who starts off as smart or intuitive says 'I don't know' or 'didn't see that one coming' at the end of the story. Another character who happily declares themself 'evil' gives a kid their toy back and admitting that they 'have their kind moments'. Even how the describe a setting shows a shift in mood or perception.
Another trick I love is showing how characters foil or influence each other. Diction-wise, A adopts B's sarcasm for a moment, and B echoes one of A's quotes at the end.