Towards the end of the movie, there is a scene where Joaquin Phoenix is sitting with his father, at a table, drinking beers. In reality, this man is not Joaquin Phoenix's father, but actually Casey Affleck and Ben Affleck's own real-life father.
Joaquin Phoenix possesses the unique ability to make himself throw up without even having to stick his fingers down his throat. This gift came in handy for the scene after his disastrous rap performance where he is seen barfing into a toilet.
In September 2010 director Casey Affleck confirmed that this movie is not a real documentary, but fake. The public had been wondering ever since Joaquin Phoenix announced in October 2008 his retirement from acting in order to become a rapper, whether it was all just an act or if it was for real.
The idea for the film came from Joaquin Phoenix's amazement at the way people believed that reality TV shows were unscripted. By claiming to retire from acting, he and his brother-in-law Casey Affleck planned to make a film that "explored celebrity, and explored the relationship between the media and the consumers and the celebrities themselves".
The movie that Ben Stiller pitches to Joaquin Phoenix is _Greenberg_, with Stiller angling for Phoenix to play the part of Ivan. That part was actually played by Rhys Ifans. This is all part of the joke, however, as _Greenberg_ was already being filmed when Stiller showed up in this scene. Indeed, this scene was filmed after the David Letterman interview and Stiller's appearance at the Oscars, dressed as Joaquin.
In the defecation scene, where Joaquin Phoenix gets pooped on in bed, the "faeces" was actually a combination of humus and coffee grounds. The mixture was inserted into a tube that was taped onto Antony Langdon's (Anton) back that went down to his butt.
In September 2010, Casey Affleck was sued for sexual harassment by producer Amanda White and cinematographer Magdalena Górka. The incidents supposedly occurred during their work together on this movie. The cases were settled.
The first hint that Joaquin Phoenix was going to pull a stunt like this came in October 2008 when he announced his retirement on the red carpet of his film Two Lovers (2008) which he then proclaimed to be his final performance. (Ironically, Phoenix got some of the best reviews of his career for his performance in the film.)
Phoenix originally conceived of the idea after he became fascinated by the show Celebrity Rehab with Dr. Drew, and in fact originally attempted to execute the stunt by being featured on the show. However, Phoenix ditched this idea when he realized it would be cruel to be performing a character amongst people with very real problems.
Magnolia Pictures were rather annoyed with the whole set-up as they were trying to publicize Two Lovers (2008) with a star that refused to be interviewed and who yet was being followed by a documentary crew at all times. They believed that Phoenix's "stunt" took a lot of attention away from James Gray's well-received film which was a box-office flop.
The final sequence in the film, where Joaquin Phoenix is trekking through the river, was actually longer in the original cut. It was almost another three minutes longer, but Casey Affleck thought that it was "beating a dead horse" at that point and decided to cut it shorter, and that is what we see in the final film.
In the scene where Anton is playing the piano and singing a song (the scene between calling the prostitutes and them actually coming), the shot in the film was actually the third take. Also, the song being sung was written by Anton's brother, and he (Anton) is singing it in a different pitch in the film.
The trivia items below may give away important plot points.
Although it might appear otherwise, one of the more enthusiastic participants in the film who was in on the joke was Sean Combs. He only had to have one meeting with Joaquin Phoenix and Casey Affleck to agree to take part. All his scenes were improvised.