For me, I usually begin with the idea of a story. And when it expands and feels large enough to move to the next step, I take a look at the people who would live that story—who are the ones to make it come to life.
When I get to know my characters, the story blossoms. If I don't get to know them, the story falters. When my words feel flat, that's a signal for me to look at a few things, chief among them my character studies.
I have a three-page list of things I use to develop the people who populate my manuscript. It's a compilation of several, plus things I've considered on my own. I'm happy to email it to you if you'd like.
What's important to any character study, or list of traits you use, is that you don't go down the list and answer each question. Yuck. Talk about boring. What I suggest you do is have the list out and handy while you practice writing about your character in a stream-of-consciousness style.
To further understand the character you're getting to know, write some of that character study in first person. It's amazing what you'll learn, and the subplots that will become possibilities.
What about you? What things do you like to do to get to know your characters? Do you get to know them before you begin writing, or do they get more fully formed as you go?
CR: Snake Skin by CJ Lyons.
It's all better with friends.