Keep asking your characters why. Here's an example:
- Why are you grumpy? I have a hangover.
- Why do you have a hangover? My friend was in a bad accident and I thought he might die?
- Why did you think he might die? His girlfriend lied to me about how serious the accident was.
- Why did she lie about that? She's jealous of our relationship.
- Why? I think she's insecure and has trust issues.
Do you see how much that question will dig into a character?