Recently, for a play, the director had a do an exercise for delving into our characters. It was the basic exercise: how do you feel about x,y, and z other characters; what are your character's religious feelings; what does your character like to do. But the one question that really stood out for me was, "What is your character's secret?"
And when we went over our answers with the other members of the cast, we were told to keep the answer to that question back. We aren't to tell the other actor's what our character's secret is, but hold it back for use in developing our characters. I think this is awesome.
I guess this struck me, because my current WIP deals a lot with secrets. My MC has a secret website and secret identity. She knows a lot of people's secrets, some of which she keeps and some of which she blabs to everyone. She's secretly in love with someone. Most of the people she interacts with have secrets, not all of which she knows.
But I think the various characters' secrets influence their actions, just like all of our secrets influence ours. If a guy has a crush on a girl he has no hope of getting with, he might not tell people he likes he, but then may take an inordinate interest in her affairs. If a girl has been blowing off her friends to hang with some other friends that her friends don't like, she might be careful to imply days in advance that she'll be too sick/tired/busy to go out with them like she'd planned. Not to mention the effort and stress that goes into the maintenance of a secret.
Everyone has secrets. While these secrets might not come out at any point in the story (though that's a surefire way to find some sort of drama to play out), it's interesting and useful to think of what they might be and what that would mean for the character.
Do your character's have their own secrets? What's your character's secret? I promise I won't tell them.