People aren't static. Everything we do changes us. Everything we witness, everything that happens to us.
Yet often times in books, it seems that the characters are static. Flat. They don't change. As readers we anticipate a change. We watch the cause on one screen and turn to the next screen to see the effect. If there is none, we feel a huge let-down.
The relationship between your characters is changing, evolving. Have you noticed? How well do you really know your characters?
My characters have a huge back story. Most of it is never revealed in the book. At one point it was revealed, but most of those scenes have been deleted. And it doesn't matter. My characters still react to things in context of their histories. The little reveals add depth to the novel, they add mystery and intrigue. Just like our relationships with our friends in real life. When we meet someone, they don't hand us a pamphlet with their back story (though heaven knows, sometimes it would be nice). We get to know them little by little. We discover their eccentricities and what makes them tick, and sometimes we never quite find out what it is about brown socks and rabbits that sends them into a tizzy every time, but we know it does.
It's okay to mess with your characters. Do something crazy, just for fun. Write a scene where one of your characters breaks a personal moral standard. What does that do to your character? How does it affect the relationships with the other characters? Write a scene where the character witnesses a crime. Loses a sibling. Gets a scholarship.
Get to know your characters again. It's amazing how dynamic they are. Maybe someday I'll be ambitious enough (and organized enough) to do a blogfest on this topic. In the meantime, feel free to share your scene with me. Just post it in the comments. I might do something with them later.