... by torturing them.
And I don't mean physically torturing them in your story. That is unless, what you have planned. If that's the case, how does said torture change their psychology? Are they paranoid? Are they coping in some way? What do they do to deal with every day life after the torture has been done? Do they rock back and forth in a corner?
Think of what you want for a character, then twist it all around. Make your character suffer. What is it that really gets to them? What are their ticks? What subtle thing do they do which they don't notice themselves doing? Everyone has something.
Make things that could go wrong in their lives, happen, and watch how they deal with these issues. Remember, perfect characters, are boring characters. No one walks away from being in a car accident without some form of shock.
Hell hath no fury like a writer with a blazing character.
Have at it people! :)
