Biology. Most of our life is automatic. We breath without thinking. Our heart beats and we blink without a second thought but when it comes to writing don't forget the body.
Physiologically we react to certain things in a very physical way. Describe what a persons body would do under these circumstance (and I suggest cheating and googling it)
Surprising isn't it how our body reacts to all these things. You can use these in all sorts of other situations. The next time you write something wonder how the body reacts. You can add in things like depression and giving birth or dozens of other things we go through in our life time we go through it without thinking about it and a little research forces us to look at it differently.