Eileen is a movie that seems like one thing — until it becomes something else. The film hinges on a huge twist about two-thirds into its run time, as meek, mousy Eileen (Thomasin McKenzie) realizes that her glamorous colleague Rebecca (Anne Hathaway) has invited her over on Christmas Eve under false pretenses. In fact, the dirty kitchen where the two are drinking red wine out of coffee cups isn’t even Rebecca’s. It belongs to the Polk family, whose son Lee (Sam Nivola) is detained in the juvenile prison where Eileen and Rebecca work.

Rebecca is convinced that Lee — who’s serving time for murdering his father — committed the act because his father had sexually abused him many times, over many years. Worse, she thinks that Lee’s mother Rita (Marin Ireland) knew about the abuse and facilitated it. To confirm her hunch, she’s got Rita tied to a pipe in the basement, and she needs Eileen to help her extract a confession.

In the film version of the story, this information lands like a brick that’s hurled in from off-screen, smacking the viewer between the eyes. In Ottessa Moshfegh’s novel, however, Eileen suspects that something is amiss before she arrives at the house. Rebecca and Eileen have a conversation about the Polks earlier in the book that doesn’t appear in the film, for one. And Eileen has a feeling as she drives to the address Rebecca claims is hers: The neighborhood is far too shabby for a Harvard graduate like Rebecca, Eileen thinks. She justifies the many red flags she observes when she arrives at the house — Rebecca must be jumpy from diet pills; she’s new in town, and hasn’t had time to clean her furnished rental — even as she knows, deep down, that Rebecca is lying to her.

Eileen plays along anyway, so the outcome is the same. But the nuance between going into a situation knowing that something is off and being blindsided by that same situation has implications that subtly change Eileen’s character and motivations. The first is more fatalistic, more morbidly curious; the second is more naïve.

It’s one of several subtle differences between how Elieen’s shocking ending goes down in the novel and in the film version. Some, like the approach to revealing the aforementioned twist, are more about finessing tone, and can be credited to director William Oldroyd. Others are in the text itself, and are changes made by Moshfegh and her co-writer (and husband) Luke Goebel in the process of adapting Moshfegh’s novel into a screenplay. Each of them affects the characters and their actions, in ways big and small.

The Kiss

Moshfegh’s novel is not without queer subtext, but one has to read between the lines to conclude that Eileen is romantically drawn to Rebecca. The Eileen of the novel fantasizes about sitting next to a warm fire with Rebecca and the two of them declaring their love for each other. But she doesn’t seem consciously aware of the possibility that those feelings might be sexual. Even in her fantasies, Rebecca kisses her hand and says, “You’re my best friend.”

In the movie, this subtext becomes text when Rebecca kisses Eileen as the two of them drunkenly smoke cigarettes outside of X-Ville’s only watering hole. This adds a new layer to Rebecca’s statement to Eileen that she “[doesn’t] live like other people” and strengthens Eileen’s motivation to follow Rebecca wherever she may go. It’s a more dramatic and explicable motivation for what follows than the vague sense of misanthropic loneliness that powers the book.

The Shot

One of the biggest changes between Eileen the novel and Eileen the movie is that, in the novel, it’s Rebecca who pulls the trigger and shoots Rita in the chest. In the film, it’s Eileen, whose trigger finger seems to move on its own, without any thought on her part. This change is also about the character of Eileen: Throughout the film, sudden jolts of intense violence flash across the screen, visual interpretations of Eileen’s suicidal ideation and homicidal hatred of her father. When she shoots Rita, these intrusive thoughts break through from fantasy into reality — a darker view of the character than the passive scapegoat of the book.

The Accused

Although the movie is structured around the reveal that Rebecca has Rita Polk tied up in her basement, its turning point is the powerhouse monologue from Marin Ireland that occurs in the next scene. The same speech, in which Rita tells her side of the story — a pathetic, disturbing justification that’s laced with resignation and self-loathing — appears in both the movie and book, almost word for word.

The monologue talks around certain blunt truths, as Rita confesses to giving her son enemas at bath time “for everybody’s sake.” She never uses the words “molestation” or “rape” directly, leaving the audience to realize with horror what exactly she’s talking about. But as with Rebecca’s big reveal, the details of the Polk family’s dark secret are foreshadowed more extensively and laid out more explicitly in the book. (“Anal penetration,” Rebecca solemnly says when Eileen asks her what Lee’s dad did to him.)

The Bathroom Breaks

One blunt and disgusting thread in Moshfegh’s novel involves Eileen’s laxative-induced bulimia. Multiple passages describe Eileen “shitting her brains out” (as she puts it) in the spare bathroom in the basement of her home, and lying on the cold floor feeling pleasantly empty and lightheaded afterward. It’s not difficult to see why this particular element was cut from the movie: Sure, it underlines Eileen’s self-destructiveness, but so do the flashes of violence mentioned above. And those don’t involve diarrhea.

The Escape

In the film version of Eileen, two shots reveal Rita Polk’s ultimate fate. First, we see her unconscious body limply banging around the backseat of Eileen’s car. Then, a second shot filmed from a partially obscured low angle shows Eileen slamming the door and walking away as the car fills up with smoke.

We’ve heard throughout the film about how Eileen’s car has a broken exhaust pipe, and that she has to drive with the windows down in the Massachusetts winter so she doesn’t pass out from inhaling the fumes. Now the windows are up, and we are left to put the pieces together ourselves: Eileen is getting rid of the evidence by allowing Rita to die from carbon-monoxide poisoning before she bleeds out from her gunshot wound. It’s compassionate, in its way.

This sequence of events is corroborated in the book, which explicitly confirms that Eileen kills Rita Polk after Rebecca shoots her. Either way, Eileen is a killer when she gets into the cab of a passing 18-wheeler and hitchhikes out of X-Ville toward — well, anywhere else. The movie ends with a static shot of McKenzie as waves of giddy relief pass over her face. It’s the same ending as Texas Chain Saw Massacre, in which “final girl” Sally (Marilyn Burns) climbs into the bed of a pickup truck to escape Leatherface, and the camera similarly lingers as similar emotions overtake the character. Here, however, that catharsis is compromised.

Rebecca

One detail that stays consistent in the book and the film is that Rebecca disappears from the narrative abruptly, at the same time, in both versions of the story. In the book, she’s letting Eileen take the fall for killing Rita, forcing Eileen to finally take action and get out of X-Ville for good. In the movie, her role in the crime is a little more ambiguous. Sure, she could be prosecuted for kidnapping. But Eileen would be the one to go away for murder, and Rebecca could play dumb and blame the whole thing on her less savvy friend.

That’s assuming that Rebecca actually exists. Rebecca could also be a Tyler Durden–type figure, a projection who appears as the person Eileen wants to be and gives her permission to act on her darkest thoughts and impulses. This theory is complicated by the fact that Rebecca interacts directly with, and is seen and acknowledged by, other characters in the film. Again, Fight Club offers a hint here: In that story, the unnamed narrator is later revealed to have performed all the reckless actions attributed to Durden earlier on.

Does that mean that Eileen, perhaps in a manic fugue, started affecting a glamorous, college-educated sense of superiority before abducting and killing the mother of one of the boys in her care at the prison? If that’s true, then the “Rebecca” who shoots Rita in Moshfegh’s novel is — also Eileen. It’s a radical interpretation of the story, but a possible one, especially considering that Eileen, who narrates the novel decades later, says that she never saw Rebecca again.

Eileen’s Book-to-Movie Differences and Ending, Explained