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).