P = Parameters. I know it can sound odd but parameters can help your characters flourish. You need to know what they would never do (or more likely say they would never do). Ideally your story will then put them in a (for them) nightmare situation where they do have to face the prospect of having to do what they said they wouldn't (or die!). They then have to make the choice of what they really would do to survive and in that you can test how well their boundaries hold up (or otherwise). You will also then know for certain what their boundaries really are. It would, I think, have to be a life or death situation to make the characters discover what they are really capable of, though of course, it might not necessarily be for themselves. What would they do to save someone they really cared about, say? What is so important to them they really wouldn't sacrifice for anything or anyone?
Q = Questioning. Curiosity killed the cat, according to the proverb, but what would a questioning approach to life do for your characters? Some careers call for this attitude (journalism I think being the obvious one - there's no such thing as a reporter who isn't questioning things and people whenever they meet them). Do your characters ever put their nose into things they shouldn't and, if so, what made them do this and what are the consequences? Does finding out the answers to their questions make things worse or better for them?