Once I have a hook I think has potential - enough to spin out more than a hundred thousand words, then I start turning my attention to characters. Who are these people? Why did this thing happen to them? But the hook always comes first.
I think one of the most difficult things for anyone who's played baseball is to accept the fact that maybe the players today are playing just as well as ever.
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.