The movie producer and all-around mensch Stuart Cornfeld once told me that in a good screenplay, every structural unit needs to do two things: (1) be entertaining in its own right and (2) advance the story in a non-trivial way.

We will henceforth refer to this as “the Cornfeld Principle.”

In a mediocre story, nothing much will happen inside the teahouse. The teahouse is there to allow the writer to supply local color, to tell us what such a place is like. Or something might happen in there, but it won’t mean much. Some plates will fall and get broken, a ray of sunlight will come randomly through the window to no purpose, just because rays of sunlight do that in the real world, a dog will run in and run out, because the writer recently saw a real dog do that in a real teahouse. All of this may be “entertaining in its own right” (lively, funny, described in vivid language, etc.) but is not “advancing the story in a non-trivial way.”

When a story is “advanced in a non-trivial way,” we get the local color and something else. The characters go into the scene in one state and leave in another. The story becomes a more particular version of itself; it refines the question it’s been asking all along.