I’m creating a character for a tabletop campaign I’m going to play in, and since the characters are pretty similar, its more important than usual to come up with a proper personality. The Elfquest roleplaying game (2nd ed.) from 1989 by Steve Perrin features this excellent table of personality traits.

What I do is go through the list and think whether a given trait applies to the character or not. Usually, new ideas appear along the way, often utterly divorced from the list itself. Eventually, I have a pretty clear idea of who my character is and who he’s not.