I don't begin a novel or a screenplay until I know the ending. And I don't mean only that I have to know what happens. I mean that I have to hear the actual sentences. I have to know what atmosphere the words convey.
Write the kind of story you would like to read. People will give you all sorts of advice about writing, but if you are not writing something you like, no one else will like it either.
I know from the middle distance I give off the look of being prolific, which is a funny compliment to receive.