Like most film adaptations of novels there are differences between the screen version and the source material of Fight Club. While screenwriter Jim Uhl's adaptation of Palahniuk's book is remarkably faithful, there are key plot points in the novel that were changed for the film version.
The ends of both the film and the novel differ drastically. In Palahniuk's novel, the Narrator does shoot himself through the cheek, just as Edward Norton's Jack does in the film. What happens next in both stories is, however, quite different. In the film, Jack is reunited with Marla, seemingly rid of Tyler forever and ready to begin to discover a life with Marla. In the novel, the Narrator shoots himself but then wakes up in what he believes is Heaven. Palahniuk's depiction makes it clear that the Narrator is actually in a mental institution. The Narrator believes his psychiatrist to be God. The Narrator continues to see orderlies and other employees in the institution with tell-tale bruises and cuts. They tell him that they are eagerly awaiting his return.
In the film version, Jack is able to reach a middle ground between the two personalities. He "absorbs" Tyler upon shooting himself and is able to become the dominant personality again. As in the novel, it is Marla and his concern for her that allows him to break free of Tyler's hold. However, in the novel it is unclear if the Narrator is truly free of Tyler. Some scholars have suggested that the last chapter may even be narrated by Tyler instead of the Narrator. The implications of such an ending in the film may have been seen by the filmmakers to be too depressing for movie-going audiences.
In the novel, Tyler intends to blow up the Parker-Morris building so that it will fall onto a nearby museum. This is intended to be a "theater of death" to kill the Narrator. In the film, Tyler has planned the detonation of several buildings all of which serve as credit card company headquarters. His intention is to erase the debt record. While Tyler's intentions in the novel are purely destructive, his film counterpart seems to have a more political agenda in mind.
Tyler himself is slightly different between the two versions. While much of Palahniuk's dialogue is intact in Uhls's screenplay, the film version of Tyler Durden is more charismatic. Evidence of this abounds in popular culture. Tyler Durden has become a meme unto himself, a symbol of rebellion. There are clothing lines that emulate the character's wardrobe and even soap that resembles the pink bars of soap in the film. Fincher himself has admitted that he wanted the audience to like Tyler but not to miss him when he is gone. Tyler's commitment to his ideology is still strong in the film but Pitt's Tyler is not quite as dark as Palahniuk's. The passage in the novel where the Narrator spots a jawbone in the garden behind the house and hides it from Marla is omitted in the film. When Tyler describes the hunter-gatherer/pre-agrarian society he wishes to return to there is the impression that Tyler vision, while still dangerous, is ultimately intended to free people. In the novel, Tyler gives almost the exact same speech to the Narrator but it is clear that this new world will be led by Tyler. One area in which both Tylers are identical is in their intention to eliminate Marla. In both versions she is seen as a threat that could undermine Tyler's very existence. If Jack or the Narrator are able to be with her, neither will require a Tyler Durden in their lives.
However, the homoerotic overtones between the Narrator and Tyler in the novel are still present in the film. Palahniuk has expressed his approval of this in interviews. The camaraderie between the two men does enter an awkward love triangle. Jack becomes very jealous of the fact that Marla is interfering in his life by taking up Tyler's time. Uhls also wisely makes the character of Angel Face a victim of Jack's jealously. Jack beats him mercilessly in the film as the Narrator does in the novel. In the film this attack is triggered when Jack sees that Angel Face is lavished with some praise by Tyler.
The depiction of Marla in the film closely resembles Palahniuk's. However, the film provides no background on her character. Palahniuk gives Marla a work background in funeral homes though the film gives the viewer no such context. The chapter in the novel in which the Narrator consoles Marla about the lump she found in her breast by telling her an embarrassing anecdote about his past is not visited in such detail in the film. On screen, Jack visits Marla and performs an awkward breast exam before leaving her hotel room. There is no real intimacy between them. Some have criticized the film's depiction of Marla as being less interesting or fleshed out. These critics see Marla in the film more as an object or goal of Jack's affection (and Tyler's abuse) than as a fully realized character.