Cary left this question on the Facebook page a while ago:
"I'm writing a horror screenplay. In watching a lot of horror movies, I find that so many of them have fantastic setups, but often fall apart in the third act. Any general thoughts on how to avoid that?"
I struggle with this answer a bit because Cary didn't offer any specifics as far as his assessment of "fall[ing] apart in the third act." Without knowing precisely what I'm responding to, it's hard to come up with an effective course of action.
There's an old joke: "The problem with the third act is the first two acts." Essentially, the reason an ending might suck is because there's not been an effective set-up for any sort of destination. A film might have a cool set-up and it might even have some engaging set-pieces to keep things going in the second act, but if it's all smoke and mirrors, it becomes tricky to conclude something that was lacking in depth to begin with.
This is why a lot of guys like me say you need to know your ending before you start writing. You wouldn't jump in your car and just start driving for days on end without planning your endpoint. If you don't have a destination, how can you develop any sort of route? Sure, you might end up somewhere, but without knowing that at the outset, how can you stumble upon the best path to that endpoint?
The horror movies that fall apart in the end are often the movies that don't have much going for them beyond a cool hook. I think I've complained before about The Unborn, which starts off with a clever idea about a college-aged girl being haunted by the twin she never knew about because the baby died in the womb, strangled by the girl's umbilical cord. Eventually we learn that the twin's spirit is trying to be born into the world. Along the way, it terrorizes the girl, killing several people around her and eventually going so far that the girl, played by Odette Yustman, submits to an exorcism.
There's some interesting use of Jewish folklore here and for a while the film holds together. And then we get to the ending. Remember - throughout the film, the main girl has been haunted, terrorized, threatened and attacked by this evil spirit. We're left with the impression he's trying to kill her. The final scenes involve Odette Yustman's character learning that she's pregnant with twins, presumably from a sexual encounter with her boyfriend shown early in the film. It's been a while since I saw this, but I remember feeling that there was a pretty clear implication that the spirit was going to be born into one of those babies.
Wait, what? So this spirit's been doing time in Odette's womb since early in the first act. Presumably, all it has to do is wait out the next nine months and he can pop out alive. Maybe - MAYBE - if Odette was considering having an abortion, the spirit might decide drastic measures are needed to sidetrack her, but she doesn't even know about the pregnancy until the film's final minutes. Given that, why the hell is the spirit pulling all these parlor tricks that will lead Odette on a path to figuring out the truth, and why on several occasions does it all but attack her, placing her life in danger?
I want writer/director David Goyer to explain to me what his thinking was here. Maybe I misunderstood some critical plot point, but the script leads the audience by the nose in so many other places, I doubt I'm too far off base. I chalk this one up to Goyer being taken with the idea that the girl not only fails to banish the spirit, but that (in a Twilight Zone-type ending) she'll actually give birth to it. In this case, the third act falls apart because the writer fell in love with a clever idea that doesn't work within the logic of the film's first two acts.
Overall, the key to not having a third act fall apart in a horror film is the same as not having any film fall apart in the third act: have a story to tell - not just a cool hook or premise. If the third act sucks, it's often because something in it isn't meshing well with the rest of the script
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