Did I say "outline"?
I recently handed in a first draft of a feature screenplay that I was hired to write by a production company that churns out about 20 films a year for Lifetime TV. I was initially given a one-page synopsis. From that synopsis I had to weave a detailed story that would fill a minimum of 95 pages.
Thinking back on that one page, I couldn't conceive that I would be able to reach that page count. I had to lean really heavily on my imagination; all ideas and angles were considered. In addition I was (initially) told: "Five characters, two locations, max." This put further restrictions on me.
I ended up writing four versions of the outline. The first one didn't hit all the marks the director had in mind (it was his original synopsis/idea). The second one was far too detailed and I was told I had to make everything into bite-sized chunks that the executive producer could easily digest.
As this is the fourth project I have done for this production company, I knew they would make the project. I also knew that if I didn't come up with something they liked, I would be replaced by a different writer.
I personally found it very difficult to keep streamlining the story points, because the outline was not just for the executive producer; I would have to follow it as well. Nonetheless, I boiled the story down to its minimum components and the outline was approved. It was 13 pages long and moved quickly and concisely.
From there I dove in (and this is a movie about swimming!) and was able to hand in a first draft in about 8-10 days, 96 pages.
Other than the outline, here are a few things that may help you in your quest to churn out a screenplay quickly (if such a situation arises for you): Make sure you complete a thorough CHARACTER QUESTIONNAIRE. Without doing this, I would be wandering around in the dark. Once I complete the questionnaire, I know who my characters are and where they are going (or not going).
Also, do not go back to the top of the script and re-read what you wrote until you are finished (unless you truly need to check or verify something). You will start fixing and playing with words and phrases instead of focusing on the task at hand: finishing the script. I have heard the phrase "vomit draft" -- which I personally abhor, although it does convey what needs to get done. But I think "first draft" is a lot more elegant, and the word "draft" means (among other things): "a plan, [rough] sketch, outline, blueprint, preliminary version."
That works for me. If you keep in mind that no matter how much preparation you do, a first draft will most likely need to be rewritten, modified, etc., you can free yourself to write whatever comes to you (keeping your outline and questionnaire points in mind), and you will know that you have made your best effort.
Do not underestimate the power and value of an outline. If you are writing for yourself, I would take that outline just as seriously and, probably, make it extremely detailed. I would submit that FIRST to my trusted peers and see if the story works. If you get thumbs up on your outline, you can move to script with some level of confidence. Would love to hear your thoughts and comments!