The truth of the screenplay is that it cannot be a piece of art in and of itself. A screenplay is written to be mangled, chewed up and regurgitated into a final film (preferably one that lives up to or indeed elevates the text that preceded it). One indication of the premature nature of the screenplay is the Image System, a term coined as a catch all to refer typically to a motif repeated throughout a film, most often comprised of what we consider characteristic of filmic storytelling; image composition, editing technique, subliminal sound cues, etc.
In short: those decisions that tend to be made long after the screenplay (which is “meant” only to describe the necessary story beats, action and dialogue) has been submitted. Such a motif is repeated throughout the film and develops associative meaning beyond what is garnered from the plot alone. You, as writer, may feel that the best way to convey a character’s duplicitous nature is to cast the parallel shadows of window blinds across their face, and to continually spell their motivation through such obstructive lighting, but most would have you strip all such creative decisions from your screenplay. Information like that should be gleamed contextually from what you’ve written regardless and be left to the director to communicate as they see fit.
But image systems can be vitally important to a film in helping to communicate much without having to spell such things out in the text, a practice which often leads to on-the-nose, illogically motivated and groan-worthy moments. An example of circumvention: In Eternal Sunshine of the Spotless Mind, directed by the wildly visually inventive and singular filmmaker Michel Gondry, plenty of shots are hazy, close up, and filmed with shallow focus, to suggest that they depict memories rushing to the fore suddenly. The script makes note of these shots as “Vague shots of…” whatever memory. In this way, writer Charlie Kaufman had put readers in mind of the film’s visual interpretation of a memory, before pre-visualization had even begun on the film. These descriptions don’t stand out as getting ahead of themselves in the slightest.
All imagery in a film has its geneses in the material, but if you want to suggest moments the production might prefer to interpret through their own lens without putting them off adapting your script, you’ll have
to be discreet about it. After all, it can be downright necessary to envision and write for a final film you have in mind. Movies come alive in elements like that, and your screenplay could be less tantalizing without a preemptive image system to help suggest its cinematic potential.
So, focus on placing descriptions which stand a better chance of not brushing up against a reader (read: subtle, implanting your ideas almost subconsciously). To know whether you’ve gone too far in minimizing your vision of the image system, Robert McKee suggests an exercise wherein you remove the image system from your screenplay and compare this version to the original. Does it loose something? Run it by a friend to be sure. A truly cinematic screenplay leaves you captivated by its story, but dying to watch it as a film.