Erum, great question.

Erum, great question. This is why it's important to determine what your story means, what some call the "central theme." If you do know it, you can use it as a lens through which to look at and assess everything else: scenes, characters, dialogue. You can use that sort out all the material you've brainstormed to determine what's relevant to the script and what's not.

Second piece of advice: Tease the reader. Or per Billy Wilder: "Don't give them 2=2=4. Give them 2+2." Make the audience work to figure out certain aspects of the story. Hold back some of that exposition. Don't include that flashback. Cut those lines of dialogue. The tendency often is to spoon feed everything to the reader. Don't do that. By omitting some narrative material, you invite the reader to become a participant in the unfolding story because they have to use their imagination to stitch together things that happen off screen.

Finally, this. Screenwriting is an iterative process. You write a draft. See what works. Maybe you need less of the "iceberg" in the script. Maybe you need more. But often you'll discover "less is more."

Hope that helps!