When you’re writing a novel (or short story) it’s really important that you know your characters very well. This goes for your protagonist(s), as well as the supporting cast. You need to know how your character would react to a given situation. You need to know the thoughts that would go through that character’s head. You need to know that character as you know yourself.
Spending the time before you begin writing really getting to know your characters will make your job later much easier.
Here are three of the exercises that I sometimes use when I get stuck.