There are two dilemmas that rattle the human skull: How do you hang on to someone who won't stay? And how do you get rid of someone who won't go?
God may not play dice but he enjoys a good round of Trivial Pursuit every now and again.
Once a novel gets going and I know it is viable, I don't then worry about plot or themes. These things will come in almost automatically because the characters are now pulling the story.