Akira Kurosawa said the best way to write is one word at a time until you get to a page for the day. The next day you return to it and do another page and you keep going as many days and hours and weeks that it takes until you reach THE END or any kind of ending...but always write until you reach the end of a story you started and put time into.
When you are stuck, make an outline of what PLOT POINTS you want to hit and then simply sit at the desk and write like it is your duty to fill a quota and set that quota (example: must write 5 pages today), not to reach artistic catharsis but a simple quota like a screenplay FACTORY...this is the key to writing success. You really do not want to be too precious about things, write like the words must be there by the end of the day regardless. In reality who cares? This is a first experiment in a series of many experiments.
Editing rough drafts comes after a speedy first draft version is ended. Some fun tools can aide this like coffee, tea or music. Put on some music and have fun with this process, the more enjoyable or energizing the music is the better. Music that fits the mood of the script can also be great. I one time wrote an entire feature film screenplay in 7 days only listening to the beautiful soundtrack from THE HOURS by Philip Glass, another feature I wrote while only listening to the song TIME from Hanz Zimmer.
This speedy/rough first draft works for a simple reason: when you have SOMETHING ON PAPER you can assess and discover the true story as you begin to fix it. Typically it happens like this "Well these guys are not working but different characters could have this conversation, this very same one and the story or scene would exponentially improve." Tricks like this lead to significantly improved drafts and excellent changes to enhance story.
The truth is that the really best writing and character development, dialogue and story points come from writing and finding the characters and story as you write. Outlines in the beginning or for individual moments can take the plot pressure off...then you should write stream of conciseness until you hit the quota and track the plot points all the way on the adventure to...
THE END.