So, Chuck Palahniuk. Power animal? “Oh, the penguin.”

Historical figure? “Jesus would be good. It isn’t fighting in the traditional sense. It’s more consensual – exploring power through an organised kind of S&M. Jesus would understand that, because spiritually he was into endurance and asceticism.”

Palahniuk is where it all began. “I read scripts all the time,” says Pitt. “And after a while you just start seeing the same thing. Then, out of nowhere, comes this voice: Chuck Palahniuk.”

Fincher seems genuinely awed by his talent; his “beautiful prose”. “To me,” says Fincher, “the movie is 60-70 percent of what the book is and that’s as much as I think you could do in 1999 in Hollywood.” Says Palahniuk, “I actually wish they’d taken more licence with the book and surprised me a little bit more.”

Not that he doesn’t love it. “It’s raised the standards and made me disgusted with most movies!” And he has a line on why, perhaps, the likes of Walker and US critic Roger Ebert (who called the film “cheerfully fascist”) were so down on Fight Club.

“It strikes a chord with young men, but tends to frighten older men,” he says. “They have the power, but they’re not ready to give it up. They recognise the world they’re moving into isn’t their world, and that’s gotta be scary.”

Palahniuk took a backseat with the adaptation.

“My editor told me not to get excited when it was optioned because only two percent of books are ever optioned and only two percent of them ever get made into movies. I had some conversations with the screenwriter, Jim Uhls, but I thought I would just fuck it up if I tried to control it.”

He did, however, visit the set, taking along some of the real-life inspirations behind the book’s unforgettable characters.

“I briefly went down,” he says. “I took a handful of friends who met the actors who were playing them. ‘Tyler Durden’ now lives in Bend, Oregon."

"He’s a carpenter. He was a rebel who wasn’t sure what he wanted but knew that he didn’t want what he was getting. He was ready to fight everything just so he was fighting. Just a big bundle of anger and angst.”

And Palahniuk helped the actors, whether he remembers it or not. “I spoke to Chuck,” recalls Bonham Carter. “And I got a feel for the person who inspired Marla and I read the book in and out.”

The other touchstone was an idea from costume designer Michael Kaplan. Recalls Fincher, “He said, ‘Here’s who she is’ and showed me a picture of Judy Garland. I was like, ‘Run with it, it’s a great idea.’ We’d call her Judy, just out of fun. Or Liza. But mostly we called her Hells. ‘Hells, daarrling!’”

Kaplan wasn’t the only unlikely voice, with Cameron Crowe having a somewhat surprising, but crucial, influence on the script.

“I talked to Cameron,” says Fincher, “because we had problems with Tyler. And he’s like, ‘It’s easy! The real problem with Tyler is that Tyler knows the answer."

"You’ve got to take out that Tyler knows the answer, so that every time somebody says to him, “My life’s fucked up, what should I do?”, instead of him saying, “Well you do this” you have him say, “I don’t know, I don’t know your situation, I don’t even know you, but if it was me, I’d try this, because at least you might learn something, even if it’s painful.'”

Screenwriter Andrew Kevin Walker (Se7en) was drafted in for the changes – about 20 percent of the script, by Fincher’s reckoning (“Jim had done all the fuckin’ heavy lifting”) – but the Writers’ Guild of America denied him a credit.

Hence, the three detectives who try to castrate The Narrator are credited, ‘Detective Andrew, Detective Kevin, Detective Walker’.

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