My question pertains to the movie by David Fincher based on the book. The protagonist has Dissociative Identity Disorder. Can you give some examples of how you feel the movie portrays this mental illness accurately, and also some ways that this movie makes a mockery of the illness—ways the illness is portrayed unrealistically—only adding to the already negative stigma attached to the disorder?

In David Fincher's adaptation of Chuck Palahniuk's satirical novel Fight Club, an unnamed narrator (Edward Norton) suffers from insomnia and depression in a materialistic world. He is weak and submissive with no life direction. When he "meets" the charismatic macho-man Tyler Durden (Brad Pitt), he begins to feel free. Durden is everything he is not: confident, masculine, and free-spirited. As it turns out, Durden is simply another personality of the narrator that the narrator imagined and mentally projected. When the narrator falls asleep, Durden takes over. The accuracy in the film's portrayal of Dissociative Identity Disorder is minimal. The narrator's creation of his other personality as a coping mechanism is realistic, for most cases of DID are acts of desperate coping. Yet, what Fight Club lacks is a severe mental trauma to cause the narrator to need Durden. In most cases, DID stems from some sort of childhood trauma that becomes too difficult to cope with. In Fight Club, the only traumas the narrator experiences are insomnia and existential depression.
One way in which the film—as many others similarly do—mocks mental illness is through its portrayal of Pitt's Tyler Durden as a violent psychopath. In addition to the brutal medieval-style violence of the fight clubs, Durden starts a terrorist organization called Project Mayhem. Mental illness in film and television frequently stigmatizes mental illness as being inherently violent or something for the general public to be afraid of. This is wholly unfair to those diagnosed with mental illnesses, for the categorization and dehumanization strays from offering actual help.
Another instance of making a mockery of mental illness comes from the physical manifestation of Tyler Durden—in other words, the fact that the narrator communicates with and sees Durden as another tangible being outside of his body. At the end of the film, the narrator begins to recognize Durden to simply be a split personality of his own, another fictionalized account, for few, if any, diagnosed with DID suddenly snap into the realization of their own illness. The narrator shoots himself in the mouth, "killing" Durden. Once again, the illness is trivialized, fictionalized, and mocked in order to provide a satisfying ending. Cures for mental illness such as Dissociative Identity Disorder are not as simple as recognizing the illness and causing self-harm to "kill" the other identity.
While an entertaining and iconic film, Fight Club certainly doesn't stand as a medically or psychologically accurate testament to a real mental disorder.