First principle
The viewer's imagined experience is the final form every screenplay serves
A screenplay exists to create a specific experience in the mind of a viewer. The page, production, and screen are means toward that experience.
What it constrains
Every scene, line, image, omission, and structural choice must be justified by what it causes the viewer to perceive, anticipate, feel, or remember. Beauty visible only to the writer is not enough.
Why non-negotiable
Without the viewer as the governing object, craft collapses into page decoration, formula, or private intention.