She dressed like Amy, she colored her hair blond, she’d linger outside our house in New York. One time I was walking down the street and she came running up to me, this strange girl, and she looped her arm through mine and said, "I’m going to be your daughter now. I’m going to kill Amy and be your new Amy. Because it doesn’t really matter to you, does it? As long as you have an Amy." Like our daughter was a piece of fiction she could rewrite. 

Marybeth Elliott is telling Nick a story about a former friend of Amy’s from boarding school, Hilary Handy. At the point of telling, Nick has no idea about any of Amy’s manipulations and false reports, so he believes Marybeth when she tells him that the girl was “very disturbed.” In this scene, it looks as though Hilary is literally trying to replace Amy, and has become so obsessed with her that she’s trying to get close to her parents. Later in the novel Nick uncovers the truth: Amy manipulated Hilary into this behavior in order to frame her as a stalker.  

There are two layers of deceptive appearances happening here. Firstly, Hilary is “disguised” as Amy, and appears to be attempting to “become” her by looking more like her. That level of apparent psychosis unnerves Amy’s mother enough to make her remember this moment clearly. Secondly, the fact that Hilary does this as part of a prank Amy is playing implies that the words Hilary says here are really Amy’s words. It looks like Hilary is talking, but it’s really Amy asking her mother if she would care if she were gone, “as long as [she] had an Amy.” Amy has always felt frustrated and eclipsed by “Amazing Amy,” whom she feels is the daughter her parents actually wanted. By playing this prank with Hilary, she’s play-acting a real life scenario with a third, even less “amazing” daughter in play. 

Amy made me believe I was exceptional, that I was up to her level of play. That was both our making and undoing. Because I couldn’t handle the demands of greatness. I began craving ease and average-ness, and I hated myself for it, and ultimately, I realized, I punished her for it. I turned her into the brittle, prickly thing she became. I had pretended to be one kind of man and revealed myself to be quite another. Worse, I convinced myself our tragedy was entirely her making. I spent years working myself into the very thing I swore she was: a righteous ball of hate. 

In this passage, which occurs as Nick sits on the plane back to Missouri after meeting Tanner, Nick thinks about his own role in the deceptions in his marriage. He reflects that how Amy made him believe he was exceptional, “elevating” him to her “level of play.” Nick felt like he had to pretend to be someone he wasn’t in order to keep her interest, and he now realizes that those pretenses wore on him. Trying to seem like he was “up to the demands of greatness” was their undoing. He initially blamed her for their problems, only to later understand that his own dishonesty played a significant role. This passage, where Nick says “I” over and over again, is a moment where he realizes that his “deception” of Amy led to a cycle of blame and resentment. Ironically, trying to pull the wool over her eyes transformed him into the hateful creature he accused Amy of being. 

I can tell you more about how I did everything, but I’d like you to know me first. Not Diary Amy, who is a work of fiction (and Nick said I wasn’t really a writer, and why did I ever listen to him?), but me, Actual Amy. What kind of woman would do such a thing? Let me tell you a story, a true story, so you can begin to understand.  

As Amy introduces the fact that her diary was a fake to the reader, she explains the cunning trick by differentiating between "Diary Amy" and “Actual Amy." For most of the novel, the intimate revelations the reader has been taking in are now revealed to have been lies and manipulations, conjured up by Amy to hurt Nick. The diary appeared to be a painful, truthful document which Amy wrote in order to cling to her remaining sanity while surviving an abusive marriage. That appearance was totally deceptive, however, and the diary itself is just one part of the disguise Amy puts on in order to fake her death. She pretended to be “Diary Amy” to everyone but Nick, so that the story would seem realistic. Indeed, she only ever drops the façade of “Diary Amy” for Nick and for the reader. She says she wants the reader to “know” her, as if she can drop the veil of deception and suddenly be trustworthy now that they know everything that came before was a fabrication.