As the protagonist, Nick is this tale's primary storyteller, but Gone Girl isn't your average, everyday first-person narrative. Gillian Flynn actually gives us a twofer—not only do we get Nick's narration of his experiences, but we get Amy's side of the story as well. If you want to get really technical, you could also say that there are three first person narrators—Nick, Amy, and Diary Amy, the voice of the journal entries in Part One that Amy admits to fabricating.
Because this is a story that mostly revolves around Amy and Nick's battle royal for the control of their marriage, having both of them share their stories with us really complicates our relationships with them as characters.
For one thing, it's tough to know whose side to take. We want to believe during Part One, for instance, that Nick really is a good guy. It's so tempting to believe him when he says that his odd behavior during the investigation is because he "crave[s] a constant stream of approval" (7.26) and can't demonstrate sincere emotion. As the person addressing us directly in Part One, we root for the idea that Nick didn't kill Amy. (We read Amy's diary then too, but it feels more like we're reading over
her shoulder than like she's inviting us in the way Nick is.)
On the flip side though, he did cheat on Amy. He took advantage of her, and he dragged her away from New York to Midwestern suburban hell—so we can sympathize with her when she exclaims to us, "He does not get to win" (32.6). Even if we think framing your husband for murder takes vengeance to a whole new—and totally unreasonable—level, we understand how she gets there. We get why she's got beef with Nick.
What we get from having these three first-person narrators isn't just complicated feelings about Amy and Nick, though—we also get complicated facts. Even when all is said and done, we don't know exactly what to think. After all—Nick and Amy have both lied to us throughout the book.
Did you notice that at various points in the book Nick and Amy each directly address us as readers? For example, when Andie enters the story, Nick tells us, "I have a mistress. Now is the part where I have to tell you I have a mistress and you stop liking me" (19.2). And when Amy reveals her plot to frame Nick, she says, "I hope you liked Diary Amy. She was meant to be likeable" (32.20). While Nick's address is perhaps emotionally manipulative—it begs us to tell him he's wrong, we do like him—Amy's is a gotcha moment where she lets us know we're along for her ride.
In drama, we call these moments of addressing the audience directly "breaking the fourth wall," but it also has its purposes in fiction. As readers we are positioned between two—or three—intensely unreliable narrators, and when they speak to us directly, we are implicated in their drama, dragged into the fight so it feels like our two main characters are fighting over us, desperately trying to get us to take sides.