3) Never write any dialogue.
Let your characters do the talking. YOU are taking 'dictation' (like a 'court reporter).
When I say this to a college or festival audience, I see jaws drop. For yes, in the nine scripts that I have written I have not (directly) written a single-line of dialogue. I set the scene with the narrative - then, I let my CHARACTERS do the talking - and I jot down what they are saying in the scene. It is like being in a trance, watching the scene unfold, and letting my characters say what THEY have to say!
Now here is the caveat. When you are writing the script, it is YOU who are the director (at-that-time)! If you really don't like what your characters are saying, or how they are saying it - then say "CUT! Let's try it again!" Then, re-write the scene until it suits you (the writer/'director').
A good screenwriter (of course) should know their story inside and out - however, trying to put words into your characters' mouths is the 'bane' of the screenwriter. Let them do the talking, and you write down what they say!
Frequently - once I come out of this 'transcendental' state of writing - and I read what has just been written, I smile - because it is like a 'spirit' has been occupying my head - and a SPIRIT it is - the 'writing spirit'!
4) Let it flow.
The STORY - especially in its *first* draft.
I wrote a manuscript for a book that I was inspired to write called "CONTRARY TO ORDINARY" (a song title from Jerry Jeff Walker), a saying that has become a central theme of my life - for I perceive myself as-being "contrary to ordinary" (the 'road less trod', you might say).
The original manuscript was 680 pages long! A couple friends of mine in Telluride read it, then suggested that I try my hand at screenwriting - because a script (they said) is only about 100 -110 pages - and recommended that it should not-be more than 120 pages (following the Hollywood axiom that one page of script is one minute of film time).
I thought "Wow!" - I can write 100 pages easily. Well, let me tell you, it WASN'T easy! The first draft of "CONTRARY TO ORDINARY" was a whooping 188 pages long! They said, "Don't worry, Mike - in your first draft just get it ALL out, because you WILL be re-writing, re-writing, and re-writing! At least you have all of your ideas down!"
And, they were right. The second draft was 148 pages. The third draft was 129 pages. And, the (presumed) final draft came in at 105 pages.
Therefore - in your first draft - just 'let it flow', and don't worry about how long it is - because a script is not TRULY finished until the film is 'in the can' - and even then there may be some re-shoots that require re-writing, or creating new pages of script! You might even say that a script is not truly-finished until it is being shown in theaters!
5) Write visually.
If you cannot SEE your movie as you write it - then, screenwriting may not be your 'calling'.
When I am at the keyboard writing - I am watching the movie unfold before my very eyes. Film is a VISUAL medium, and if you cannot see it, then it just isn't there! Once again, Allen Cody Taube is a master at this. When I am reading one of his scripts, I am WATCHING a movie - and THAT is the mark of a good screenwriter!
Novelists have-to write visually, as well - however, to do so in writing a book may take a novelist a page to 'draw the reader into' their story scene. A screenwriter must do the same-thing - in a mere fraction of lines! THAT is the challenge for the successful screenwriter.