"FIRST THOUGHT BEST THOUGHT" - Allen Ginsberg
Pamela and I attended a poetry workshop delivered by Allen Ginsberg shortly before his death. His very simple syllabus was a two-sided piece of paper containing a numbered list of short quotes, each one illustrating or signifying a point he expounded upon as he went. One of those quotes that has stuck with me ever since is the one above, from Ginsberg himself.
The Muse is right! Instinctively, artists know what works and what doesn't. I think that's a pretty good definition of "talent" -- the ability to listen to your instincts and act on them. If you just take the first thing that pops in your head and go from there, you'll eventually come to the end and have a completed script.
If you keep discarding those first thoughts, you'll never get anywhere. (This is also the difference between good improv and bad improv. Good improv takes the first thought and just rolls with it. Bad improv stands there hemming and hawing, never really starting.)
The idea is to just flow, and don't edit as you go along. If the first bit of dialogue you come up with concerns the quality of salmon at a particular restaurant, go with it. You may cut it out later, or it may turn out to be the single most important piece of dialogue ever written. If you get into the habit of editing yourself before you've written anything, you'll train yourself not to write. Action begets action, and object in motion tends to stay in motion, etc. So move! Write!
Once you're done, edit. Be merciless -- after the first draft is finished.
I've hemmed and hawed as a writer. I've gone off on wild snipe hunts in my mind, trying to second guess the Muse. Nine times out of ten, I come back to the first thought the Muse gave me and beg her forgiveness for ever doubting her.
(Usually, she lets me slide.)