Two that I especially like:
- When the story stalls, ask: what is the character thinking now? Is she thinking anything? If not, why not? Characters need to learn something about themselves, about their values and assumptions.
- Give the character something to do in the scene. It brings the character and the scene to life. A character soaking in the bathtub, thinking about her rotten marriage is boring. A character performing brain surgery, thinking about her rotten marriage is a different proposition.