What Won't You Do?

Arguably, the most important thing to know about a character is what they won't do. Anything not on the list is entirely reasonable, and anything on the list is exceedingly unlikely. 

Stories often involve people and their limits. What are their personal rules? What are their morals? How did they come up? Why do they keep them? How has that line affected their lives?

The thing to remember with these rules, though, is that they can be broken. Sometimes a story will show how the most extreme circumstances can cause a person to go over the edge. These do have to be quite extraordinary though. Otherwise it is a cheap twist that more looks to break a character than to grow it.