I took an acting for film class a few years back and I had to do this one scene in which my character was very serious and spooky and a little bit frightening. And at the end of my monologue, I smiled. My coach was surprised, but excited, that I had made that choice, and he asked me why I had done it.
I said that I wanted to take the scene, and flip it on its head - that I thought it would be more interesting and add more layers. And I had been right!
Try to do this with your characters. Find moments when they can say or do things that are a little unexpected, but still in character, to add those extra dimensions. It adds depth to your book.
What book have you read where the characters did unexpected things?