I was asked by a fellow writer how to approach characterization in a way that allows one to express their personalities without putting it into plain text. And I think that's a great question. No one likes to read about how a character acts outright. They want to see it. A lot of this will come down to the classic "show don't tell" advice. No one wants to read a passage where you say something like, "Jason was a constantly nervous person who liked ice cream when he could get it. He had a short temper and was prone to large outbursts. He hated this about himself and would cry himself to sleep at night." This is the sort of narration that might fit well in a Wes Anderson movie, but to make that character interesting without a person like Gene Hackman or Bill Murray sort to inhabit it, you need to show us all of these things. If your character is constantly nervous, how can you show that in other scenes? Are they constantly biting their thumbnails?