Image Credit: PRN/PR Photos (2); Bob Charlotte/PR Photos

Casey Affleck has already admitted that I'm Still Here, his doc chronicling Joaquin Phoenix's highly public meltdown, is a fake. There was no drug addiction, no rap career, no personal crisis: Just one big prank that Affleck and Pheonix pulled on the entire world. (They aren't just co-workers: Affleck is married to Phoenix's sister, Summer.) Phoenix himself came clean on Letterman in September, apologizing to Dave for his notoriously mumbling appearance in February of 2009.

Now, in the movie's DVD audio commentary (out Nov. 23), Phoenix and Affleck acknowledge that they weren't the only ones in on the hoax. Almost all the Hollywood stars who appear in the film — including Ben Stiller and Diddy — knew exactly what was going on.

Diddy plays a large role in the film, listening patiently as Phoenix tries to convince him to produce his album. Diddy later defended Phoenix, telling reporters that the actor's rap career was authentic. But according to Affleck and Phoenix, it was all an act. "Puff was great. He invited us over to his house, and we actually went over and we explained what the movie was to him and you know, sat with him for about a half an hour and sort of talked him through it. And all he said was that he was in," says Affleck, who did multiple takes of most of Diddy's scenes. "Puff… took direction very well."

Stiller, who memorably skewed Phoenix's persona with a gag at the 2009 Oscars, makes an appearance in one of the film's tenser scenes, when he tries unsuccessfully to pitch a movie role to Phoenix. "We actually filmed this scene after Joaquin's appearance on Letterman, after the Oscars, when Ben makes fun of him," admits Affleck. "And then we went back and we shot this scene so we could put it earlier in the movie and give Ben a reason. We had to sort of justify why [Ben] would be making fun of him that way. Give him a kind of personal motivation. And he was totally game."

Phoenix also says that his infamous onstage tumble during a rap set at a Las Vegas club was planned — and inspired by Chevy Chase. "Most of my life I've been consumed and plagued with the fear of falling or tripping in public for some reason. And any time I walk the red carpet or go on to a talk show or something, I'm certain that I'm going to fall and make a fool of myself. And I just thought that this was kind of a perfect opportunity to see the character fall," says Phoenix. "I was always a fan of slapstick comedy, and Chevy Chase always used to do this thing where he'd fall over and jump up immediately, like he was fine. And I just basically tried to steal it from him."

Other stars in on the joke: Natalie Portman and Edward James Olmos. Affleck doesn't discuss whether Letterman was in the loop, saying only, "He was great." (A Late Show writer has said Dave knew about Phoenix's plan.)

Reps for Stiller and Diddy did not immediately respond to requests for comment.

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