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Giant spoilers ahead. Do not read on if you have not read or seen Gone Girl and you'd like to do one or both at some point.

While Gone Girl was becoming a major word-of-mouth hit a couple of years ago, the selling point was the giant twist halfway through. But a recommendation of the Gone Girl would also come with a warning about the ending, which was highly divisive, even among people who liked the book—just look at any online review.

Then earlier this year, in an interview with EW about the movie adaptation, director David Fincher said, "Ben [Affleck] was so shocked by it. He would say, 'This is a whole new third act! [Gillian Flynn] literally threw that third act out and started from scratch.'" Other blogs ran with the quote, wrongly implying that Gone Girl author and screenwriter Gillian Flynn had caved to the negative response to the ending and changed it for the film. As it turns out, the third act was indeed structured differently for the film, but the ending—the one that infuriated or thrilled so many readers—was exactly the same.

EW talked to Flynn, a former EW writer, about that ending back in 2012: Amy, the sociopathic mastermind played by Rosamund Pike in the film, fakes her own death, ruins her husband's life, and pretty much gets away with it. Read on for Flynn's explanation as to why Gone Girl's ending is the only one that would have worked.

EW: The ending of Gone Girl is a big point of contention for a lot of readers. Were you surprised that people reacted so strongly to it?

GILLIAN FLYNN: You know what, I really was. It's not an ending where I wrote it and thought, "Haha! This is going to be so controversial, I've written such a divisive ending!" I was very taken aback and very surprised by the number of people who either love it or hate it. People come down very strongly on one side or the other.

What did you want to accomplish with the ending?

First of all, I didn't write it as an open ending to set up a sequel at all. It was the only thing that made sense to me, that made sense to what was true to the book and true to the characters. Amy's not going to end up in jail. She's Amazing Amy! You're never going to find the "aha!" clue because she thinks she's already thought of everything and that's who she is. People think they would find that satisfying, if she were caught and punished. You know, when I'm at a reading or something, people will come up to me and are very honest about saying, "I hated the ending!" I always say, "Well, what did you want to have happen?" And it's like, "I wanted justice!" I promise you, I just don't think you'd find it satisfying for Amy to end up in a prison cell just sitting in a little box.

I know a lot of readers wish she'd died.

That would have been the other option, to kill her off—but who's going to do that? I'm not going to have Nick do that. He's not going to do it. And to have anyone else do it is putting him back in the stage where he started out, which is having other people do his dirty work all the time, so that didn't work for me either. So you know, I did think it all through, and for me, I've always loved those endings of unease.

Right—open endings are a matter of personal taste.

Rosemary's Baby is one of my all-time favorite books. I love that it just ends with, you know, "Hey, the devil's in the world, and guess what? Mom kind of likes him!" And that's the end. I love the end of Notes on a Scandal, also a book I love. It ended in that same sort of ominous way. Things aren't going to go well. You don't know quite what's going to happen. People always want to know, "Well, what's going to happen?" Well, you know nothing good, right? So for me, one of my all-time favorite works is Who's Afraid of Virginia Woolf. I have several little inside jokes, character names, names of the town—little, little references to Who's Afraid of Virginia Woolf that no one was supposed to get. That idea that Who's Afraid of Virginia Woolf ends with this very lethally entwined couple moving into a new phase because they destroy an imaginary child, and Gone Girl had Nick and Amy, a very lethally entwined couple, also moving to a new phase. Whether bad or good we do not know, because they actually are bringing to life an imaginary baby. It was playing around with that a little bit, too.

People really just want to see a sneaky person get her comeuppance, right?

I suppose you are used to seeing the bad guys punished. We're trained from all the TV and movies that we watch to believe that the bad guys go down at the end and the good guy wins, but I do think that there have been enough anti-heroes by now at this point. We watched Sopranos for many years, and that was certainly an ambiguous ending where the bad guy doesn't go away, and you don't want him to because at that point you have an allegiance to him. Maybe that's the difference between who likes the ending and who doesn't: Those who do find themselves having somewhat of an allegiance or an affinity or some sort of liking for Amy, and those who consider her evil all the way through.

That's what puzzles me about the reaction. You can like an immoral character because she's interesting, not because you want to have her over for dinner.

We're not doing a likeability test here! [Laughs] You don't have to go on a cross-country road trip with them to find that character really interesting. And I think that definitely goes to the different reasons that people read and engage with what they read. I've never been someone who really needs a hero narrative. I don't need that person overcoming odds and becoming better and that kind of traditional arc. I've always read in order to figure out people more, and that includes bad people and good people. I want books to give me insight into the way people's brains work and hearts work, and that's what engages me. I've never thought that any type of book should have to end in a certain way. I wanted to end it in a way that feels real. Like I said, I sought different ways to end it, and that one was the one I kept coming back to that felt the way those two would quote "work out" their issues.

I do want to add, though, that it's fine with me if people don't like that ending. I've heard there's online fan fiction that people write their own alternate endings, and that's cool! If you're still thinking about the book and have your opinions on it, I think it's a great thing.