The most underrated talent in screenwriting is knowing when the screenplay is done. Not the first draft. Not the second draft. Not the draft you’re taking out to your first round of contests and not the draft you’re taking out to your writing group peers for feedback.
You may be thinking, I’ve had conversations with other humans before! Writing dialogue will be a piece of cake! Then you write a few pages and realize that your dialogue is stiff, clunky, and puking exposition everywhere.