Sure, the hero has wants, sure he sets out on the journey, and so on . . . Well, now sure that I had found my solution to focus, I tried writing out the theme in portentous prose, but it didn't quite work. Note, I said one word, not two, not three, one. It's easier to keep one word in mind than three, six, twelve, or whatever. Keeping your underlying idea in mind at all times is the key to focus, and focus is what you want.
Lets take the movie The Verdict as an example.
The Verdict comes as close to a perfect story as anything I've seen. The journey is that of a lawyer, a drunk and a sleaze, who, to get justice, takes on the entire Boston power structure. Because the hero's demand-line changes as he enters the extraordinary world. We see the sleaze, superbly acted by Paul Newman, working on a medical malpractice case. He goes to the hospital to see the young woman sent into an irreversible coma by the doctors' negligence.