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This interview contains spoilers for Gone Girl. Do not click through if you have not read nor seen Gone Girl.

Rosamund Pike finally has her breakout role with Amy of Gone Girl, a complicated woman who presents a different version of herself to different people in her life. Pike thought this was great fun to play, but also exhausting, given that she had to play multiple Amys, and the pregnant actress explained her process during this chat with Vulture, also sharing her thoughts on Cool Girls, training with a boxer, and learning to not give a damn.

So you shot the Ozarks scenes, when your character alters her looks, first …

I had to gain and lose weight for those scenes three times over! I had to be slimmer in L.A. and heavier in the Ozarks, and then heavier and slimmer in the studio, so I had to do it three times over. They scheduled around it, so I had two weeks each time for each fluctuation. It was quite intense. It was like turning your body into a chemistry lab, and I couldn’t escape it, the weight-loss period. I took time out to get a pedicure, and Extreme Weight Loss! was on the TV. I was like, “I can’t get away from it!” [Laughs.]

Is that why you took up boxing? Because it’s not like you needed to train for a fight per se…

I didn’t quite take up boxing. I trained with a boxer, [welterweight Holly Lawson]. Anyone who is going to go into the ring will usually rest over their fighting weight, and then lean out to go into the ring. So they’re very conversant in being one weight, and then having to drop in a confined period of time. So it made perfect sense, right? Because of their ability. To a certain extent, my body cooperated, but I don’t know how good it is for you. It’s probably better to be gaining weight steadily over a long period of time, as with the baby. [Laughs.]

We only get a taste of it in the movie, but Amy’s “Cool Girl” rant is the heart of the story. It gets to the crux of her sociopathic behavior, while also making you identify with her and see some truth to what she’s saying …

Absolutely. But I think that’s the thing about the sociopath: They’re often very likable people. As I read around it, the sociopath is often the one who draws people in. But frequently, one of their traits is to divide and rule, so they’ll present a completely different face to different people. You get those people in a room, and they’ll have a fight about who the sociopath actually is. “But she’s so sweet! What do you mean?” “No, she’s ambitious, and powerful, and pushes everyone out of the way!” That will be their technique for making sure alliances cannot be made and [ensuring] that they stay at the center. Which you do see Amy doing at certain points. There’s a lack of sincere and genuine emotion, but [sociopaths are] sort of fascinated by it and can perform it. That tends to be the thing I’ve come across most frequently in my research, that they’re usually good actors.

In a way, being a good actor is all about stripping away any façade. That’s the point. You’re getting at some fundamental truth. But playing Amy, Amy is so self-aware that she’s constantly … she’s never really in an authentic place in the whole film. She’s not like Margo, who is just sort of relentlessly, authentically herself. You think, If only Amy could experience what it’s like to be a woman like that!

She has a cultivated self, but not an authentic self.

Yeah! Amy, I don’t even know who her real self is. As a child, she grew up with this sort of unearned adulation or, I should say, unearned fame rather than adulation, and I think anyone who has unearned media attention, because they’re married to somebody famous or because they’re the child of somebody famous, it gives an insecure sense of self, because there’s no foundation. There’s this sort of entitlement without any dues having been paid. And in a way, that’s Amy.

That’s probably why she’s attracted to personality quizzes. She’s trying different personalities on for size …

Absolutely! I mean, she’s the person who says, “Are you A, B, or C?” And those quizzes, as you know, are very simplistic, but also kind of irresistible. We like packaging ourselves like that, like psychology tests online, which became fashionable for a while among a group of friends. And you try to be relentlessly honest, and then you think, So what am I saying here?

If you’re working with David Fincher, you’re going to do even the most simple and mundane moments over and over and over.

He’s definitely asking you to get to the point where you’re divesting all your values. All I know is if you tell an actor they’ve got to get this in the next 20 minutes, that’s a surefire way to make sure they clam up and get tense and try too hard. He’s getting you to the point where you’re just sort of rooted in being, and you’re not trying. We want to do well, so we try too hard. It is a curse, so that’s what you try to get rid of as much as possible.

Sometimes he finds inspiration in the mistakes that come from doing an action enough repeated times.

Maybe that’s more true with other actors. When something looked bungled, that’s not what he wanted from Amy. Amy has to have this precision to everything she does. So with me, it was this fluidity, this smoothness, even just getting into a car. Amy had to glide. He wouldn’t want Amy to drop her keys and fumble in the foot well. She’d have to slide in like a ballerina because she’s so self-aware of how she’s coming across. Even when there’s no one watching. She’s watching. [Chuckles.]

There was so much preparation for all this, so much preparation, in a way, that I never really clocked off. There was always something to think about with Amy. I had to transform the obvious way, and then I had wigs, I had to do the schizophrenic weight-gain/loss, I had to change my voice … Not just change my accent. Amy has specific different voices. Because there’s Diary Amy, and then there’s the other voice-over, where the vitriol comes out and you hear her critique of her own gender.

What do you think of her critique, though? Do you agree?

I think most women identify with it, and that’s why the book was so successful, because women find it sort of deliciously undeniable, something they recognize in themselves and their friends. And it’s something they’ve tried on. I’m not saying most people would try it on forever, but most people can identify at some point with having tried on other personas. It could be because you’re attracted to a guy. It could be because you meet a girl in school who you think is cool and you want to be her friend, so you take on her interests, what she’s into, and dress like her. We’re all on it. And Gillian Flynn is wonderful because she’s not frightened to put these ideas out there and open herself up to people saying, “Oh, you’re just writing about yourself.” Most people don’t want to go there because they think it’s going to reflect badly on them, because we’re all so aware of how things are going to reflect on us! This is the curse of the modern age! [Laughs.] We’re all thinking, What if I say that? Then it’s going to reflect badly! A new freedom comes if you can, you know, not give a damn about that stuff.

But it’s hard. It’s really hard to do that. That’s the thing about Fincher — here is a guy who is only interested in authenticity and truth. He’s not going to buy any bullshit he’s read anywhere. He’s just going to meet you and zero in on you, and he’s going to say, “Who are you?” Like Amy does, actually. Like when she says, “And you. Who are you?” And she’s not just flirting with the guy. She’s saying, “Who really are you?” It’s that scanner, that high-bit processor that is going off in her brain … which makes her so fun to play! But also exhausting to play! [Laughs.] Sometimes I wished I could have played Margo — not that I’m saying that that’s an easy role! It’s a complex role, but you know who Margo is, and you can just go for it.

So would you say you’re a Cool Girl? Or a Kooky Girl?

I’d just stay with Complicated Girl. Honestly, after these past three days, I have no idea who I am anymore. [Laughs.] I have to go home! [Laughs.] I just have to go home, get on the floor, and play with some Legos.