When you get a great idea and start without developing the bones of the story, you will probably get about thirty pages in before the wheels come off.
Which is fine. You can put down your pages and do your outline then. But you still need one, because screenplays are incredibly intricate. Every scene you write builds on the last one and sets up the next one.
Like this.
It is easier to prewrite than rewrite. Even if you’re like me and you don’t meticulously beat it out scene by scene before you start your first draft, you have to have a strong logline and a map through your second act.