Tip of the Week: Characters are Real People Too
Now I haven't developed that disorder where I can't tell reality from fiction. I don't mean that I take my characters out to the movies or sit down to share a refreshing cup of tea. My characters aren't the tea drinking sort.
What I mean is that all of your characters are real within the confines of your fictional world. They have pasts, presents, and (if they aren't killed off) futures that you as their creator ought to know about. These backstories probably won't make it into your actual novel, but they will help you know how your characters react. They give you a reference point for grounding each of your characters, no matter how insignifgant or minor the character is for this particular story.
And knowing this backstory might someday provide you with a whole different story to tell.