Before I talk about screenwriting, I want to talk about fractals for a moment.
Fractals are objects where the overall shape resembles the shape of the component parts. (Yes, yes, I know that’s a vast oversimplification of the proper mathematical definition, but I don’t think I need to get into a discussion of the Hausdorff-Besicovitch dimension to make my point about the three-act structure.)

Why do most mainstream screenplays–and virtually all good mainstream screenplays–end up fitting into a three-act structure?

Alex Epstein has posted some interesting thoughts on the three-act structure. This is something I’ve thought about a fair amount, so I thought I’d weigh in.
First, for those of you who aren’t screenwriters, this post is going to be a brief overview of the three act structure. I’m going to take my examples from the original Star Wars (or “Star Wars: Episode IV: A New Hope,” for my fellow geeks). If you are one of the nine sentient beings on the planet who has yet to see the film, spoilers follow. There is also a spoiler for Raiders of the Lost Ark.

(More SPOILERS for Taxi Driver follow.)
My friend James has pointed out some more info on the upcoming Taxi Driver videogame:

“The game picks up where the movie left off,” said the developer in a statement. “As Travis reminisces about his bloody rescue of the young prostitute Iris, it seems the violent catharsis and recovery that ended the film has turned his life around. However, a terrible sequence of events finds him unable to stop the murder of someone very special to him. His ensuing quest for revenge finds Travis Bickle once again on an inexorable path towards violence. Players will fight their way through the mean streets of New York City in Travis’s bid for vengeance, to bring the ruthless rain that will clean the scum off the streets once and for all.”

I was rather perplexed to learn that they’re making Taxi Driver into a video game.
Allow me the first to say: wha…?!?
This is a bizarre decision on so many levels. For one thing, I can’t imagine there’s any sort of useful brand recognition. Watching The Godfather might make you fantasize about being a mobster, but nobody who sees Taxi Driver comes out fantasizing about driving a taxi.
(WARNING: Spoilers for Taxi Driver follow)
Actually, you could make this a really creepy and disturbing game. It could be like a Japanese dating sim, where you try desperately to connect with Cybil Shephard and Jodi Foster, only unlike the Japanese dating sims, there is no right choice at any moment, and no matter what you do, you become more and more distanced from humanity until you lose the game.
Unfortunately, I can’t help feeling that the more likely result is a shoot-em-up where you righteously mow down evil pimps, thereby missing the point of the film entirely. And then we can look forward to the inevitable film adaptation of the videogame, directed by Uwe Boll…