"A writer, when he [or she] plans out a future work, establishes always a series of, as it were, key-stones, significant to the elucidation of the theme and spread over the whole of the work in preparation...
The [filmmaker, likewise] must think in plastic (externally expressed) images.
They must train their imagination, they must develop the habit of representing to themselves whatever comes into their head in the form of a sequence of images upon the screen."
- V.I. Pudovkin
That's quite a mouthful.
But what does it mean?
Let's have our good friend Mr. Kubrick expand on the idea:
"Forget it, all you need for a movie is
6 or 8 non-submersible units."
This is a practical way to express the theme of your film.
You think up, or set aside, 6-8 key scenes which clearly deals with the central idea.
The rest of the movie can be novelty, variation, permutation, extension, suspension, pure entertainment, or simply filler.