From an interview between Joss Whedon and Andrew O”Hehir of Salon.com. Whedon is talking about his recent meta-horror movie, Cabin in the Woods:
… The plot is something I presented to Drew as “I think I found the movie that we could actually sit down and write in a weekend,” because it has a third act. It starts one way then takes you another way and just when you think you know where it’s going, it goes a third way. And this is how it wraps up. And not only did I present it to him all in a bundle, but it came to me that way. The structure came first. Not, “We should make a movie about a guy named Marty.” Or, “We should make a movie about two guys in an office. What could they do?” The structure is what appeared before me, shining like a unicorn. And I went, “Oh.” And we just filled it in from there. And structure is the hardest part of storytelling.