Dyeing for Your Art: As the filming went on, Norton deliberately lost weight and slept little while Pitt exercised and got tanned. One character wasting away while the other gets stronger has plot relevance here.
When Tyler asks the narrator he wants to be hit as hard as he could, the narrator winds up and... hits him in the ear. Brad Pitt was told Ed Norton would hit him in the shoulder. "Ow! Motherfucker! Why the ear, man?!" is a legit reaction.
Well not so much enforced as it was lucky method acting: Marla's amazingly blase delivery of "I haven't been fucked like that since grade school" was the way it was because Helena Bonham-Carter is British and didn't know that grade school in America was the equivalent of primary school: she thought that meant Marla was a teenager when she first had sex. When Helena Bonham-Carter later learned the truth, she was not happy.
The scene of the Narrator and Tyler whanging golf balls out into the neighborhood was captured because Norton and Pitt did exactly that after having a lot of beer. The actors were aiming at the catering truck.
Fake American: British Helena Bonham-Carter as Marla. She points out occasions when her accent slips, such as when she says "Cornelius."
Executives felt Marla's line after she has sex with Tyler ("I want to have your abortion") would prove too offensive, so they asked David Fincher to change it — which he did, to "I haven't been fucked like that since grade school". He refused to change it back.
The movie contains a great deal of product placement, nearly all of which ends up criticized, smashed, blown up, or otherwise vandalized over the course of the movie.