Take my word for it, Screenplays are not about words or pretty writing. They're not a vehicle for dialogue. Or character study. Of course you can't have a nice house without a good paint job. For me, that's what dialogue and great characters are in a screenplay... trimmings, on a hopefully well built, structurally sound house. A house made up of what we see - NOT what we hear.
OR - if a film is a vessel, the Director is its Captain. You build the ship with your heart, sweat, blood and tears and then you push it over, across the pond, into the Director's hands and watch it get scooped up and carted away to its real home: 35mm (or whichever stock he so chooses).
It took a looonnnggg grueling walk through more than a couple of scripts for me to learn this. To put everything I have onto an 8 1/2 x 11 sheet of paper, hand it in to Producers/Directors and have them say "Great job" then change everything, killed me over and over again. Even when I thought I was already dead and they couldn't possibly kill me again, they did. Flippantly too. Why? Because that's the game. That's exactly what you sign up for when you agree to write a screenplay.
PLAYWRIGHTS are wordsmiths. The vessel IS the word that is coming out of the Actor's mouth. All we have, as audience members, are the beautifully, well constructed, pawed over sentences put down by the author. That is the beauty of a play. The words are only what we have. They are fabulous clues to everything we need to know, in order to figure out what the Playwright is trying to say. THAT'S why Playwrights get the respect they well deserve. The play IS the word. Of course you can't change a syllable!
Why can an actor step onto a movie set and change around the writer's dialogue with great abandon? Because the film is NOT the word. The film is the SHOT. If the words are good, it's a plus! But they do not matter. The shot is the thing. The shot tells the story.
Who is the screenwriter who doesn't understand this? One that will live in pain, self pity and masochistic rage each and every time he puts his screenplay down on the pond and watches it float away.