AceShowbiz - Casey Affleck has revealed his new movie about brother-in-law Joaquin Phoenix's efforts to quit acting and become a rapper is one big hoax. Affleck and Phoenix fooled fans and the media last year as the "Walk the Line" star announced he was turning his back on Hollywood and then grew a bushy beard and started acting strangely on a series of chat shows, and at appearances.

The madcap scenes were shot by Affleck for his newly-released "I'm Still Here" documentary. And during recent interviews at both the Venice and Toronto Film Festivals, Affleck insisted the documentary was for real - and his pal and subject really was trying to make it as a rapper.

But in a New York Times article on Thursday, the actor/director confessed the film was an experiment that perhaps went too far. He said, "I never intended to trick anybody. The idea of a 'hoax' never entered my mind."

But Affleck admitted Phoenix played his part brilliantly - even risking ridicule for a mumbling appearance on American chat show host David Letterman's late-night programme last year. He added, "It's a terrific performance, it's the performance of his career."