Have you ever looked at a big project, unsure how you would ever have the time, energy, or guts to finish it? It is overwhelming, unwieldy, and very, very frightening.
That is exactly what writing a novel or a screenplay is like. As a whole it appears impossible. It is too complicated, too monstrous, too hard to wrap your head around.
Which is why you write chunks. Manageable chunks, or 10 or so pages.
Anyone can wrap their head around 10 pages, right?
Of course, if you jump into writing those 10 page chunks without any planning, well, the result would be pretty much the same as attempting to build a mansion without a blueprint. You might manage to make it look all right on the outside, but no one would want to spend very much time in it.
Pre-write! Develop the characters. Create the plot. Build the structure.
And then jump right in and write chunks.
It is a lot of fun. Really!