At the very end of this video (excerpted here), the director Francis Ford Coppola explains how he chooses a single word for each of films to keep him on track as he makes decision after decision:
Learning from the great Elia Kazan, I always try to have a word that is the core of what the movie is really about.
In one word.
For Godfather, the key word is “succession.” That’s what the movie is about.
Apocalypse Now: “morality.”
The Conversation: “privacy.”
Megalopolis. You know what it is? “Sincerity.” That’s the word I use when I say, “What should I do?”
For each book I write, I choose a secret sentence.