5. Lost In La Mancha (2002)
Like a couple of other additions on this list, this documentary isn't strictly focused on the notion of screenwriting, but it is all encompassing of the massive hellfire that is making movies in general - in this case,
Lost In La Manca is centered on writer/director Terry Gilliam's attempts to adapt
Don Quixote with Johnny Depp in a starring role. For aspiring screenwriters, this movie is the only one you'll ever need when it comes to understanding just wrong things can go when you're shooting. That's to say,
Lost In La Mancha depicts Gilliam's movie like it's cursed from the very beginning - watch this, and you'll agree that there's something disturbingly unholy about all the screw-ups. Aside from the fact that the weather continually plagues production, the lead actor succumbs to illness at the most crucial moments, and the investors decide to pull out, this is a great doorway into a terrifying world for anybody who wants a honest, uncompromising window that doesn't feel like a DVD extra. You also get to see first hand how Gilliam's script is adapted for the screen, which is never anything but interesting.
Don Quixote remains unmade to this day, of course, but I've got a sneaky feeling that - ultimately - this documentary might've been worth all of the torment. Gilliam, I imagine, would strongly disagree.