I'm asked this question quite a bit so I thought I would try to answer it today. When I begin a new story I have an idea of who my main characters are but it isn't very fleshed out. It's hard to describe, but it's more of a feel for them.
Maybe this will help explain it. You know when you meet someone for the first time? Do you get a feel for who they are right away? I usually do. It's more than a first impression - more like a peek into their spirit, their heart. This isn't to say I know everything about them right away! I just get a feel for who they are and that leads me to what they might say or do and how they might act and react.
Maybe this is something that actors/artist/writers/musicians (basically artsy people) have a knack for, I'm not sure. But I take this "feel" for someone and transfer it to my character. I don't fill out those character development sheets (I tried that once and it took all the joy of discovery out of the process for me). Instead, I just go with this gut feel and then watch in amazement as they come to life. That's the fun part!!
Also, I think there is a little of me in most of my characters! LOL! Some characters, like Jane in Snow Angel, closely match my personality and how I handle difficulties and trauma. Other characters, like Serena in The Duchess and the Dragon, are more a melding of some favorite people in my life (my sister!). But the best thing is when a character suddenly becomes real and takes off on their own. It’s like having a child. You see some of yourself in him/her and then they do something or say something that totally surprises and delights you. That’s when God’s vision for the character really shines through.
I'm thinking Scarlet Johanson looks a bit like Emma, the character I'm currently creating.
What about you? If you write, how do you create your characters?