I endlessly rewrite as I write. As in, read the whole thing three times, write a little, reread, go back, read, rewrite, reread the whole thing, write a little.
It takes me as long to write one draft as it takes other people to write three or four or nine. It does come out highly polished, but it’s a paralyzing way to write a screenplay, because parts of it become cast in stone, which is always bad.