Be mean to your characters.
What they take for granted? Take it away from them.
The one thing they know for certain? Make them doubt it.
Their worst fear? Throw it at their damn faces.
Make their plans fail. Make them cry. Make them question things and fuck up and then learn about them and the way they react in the process.
Be mean to your characters. Then be kind to them for a little while, because after all that stuff they may deserve it.