From time to time I do more reading about the writing process to work on developing my skills (aside from the great stuff I find on everyone's blogs). Right now I'm reading The Plot Thickens by Noah Lukeman, since I always seem to have issues with my plots. The first chapter is about characters. It essentially helps you to create a list of traits about your character. I've never done too much of this in the past, because I always thought I'd "discover" more about my character as I write.
But here's the thing I never understood before: once you have that list of descriptions of your character, the way it really helps your story is to think about how each one of these traits could affect the plot. Time consuming? Yes. But, wow, it really helps you get a deeper understanding of your character's world.