Joaquin Phoenix transformed from a clean-cut actor into a bearded, druggy rap wannabe two years ago - but it seems it was all in the name of art. His brother-in-law Casey Affleck, who made a documentary about Phoenix's transition from acclaimed actor to bearded, shambolic hip-hop star, admitted Thursday that it was an act. “It's a terrific performance, it's the performance of his career,” Affleck told the New York Times. Affleck said Phoenix put his professional life on the line to star in the documentary, “I'm Still Here,” which captivated viewers at the Venice International Film Festival this month. At the event, Phoenix swore it was a genuine documentary despite clear indications on the screen that it was a “mockumentary” poking fun at a narrow-minded public. In the movie Phoenix is at times funny and coherent and at others childish, aggressive and paranoid as he struggles to live with his decision in 2008, which the media greeted with breathless disbelief, to give up acting and take up hip-hop. The film includes footage of him apparently taking drugs, diving off a stage to attack a heckler and vomiting. It also features rap impresario Sean Combs, who finally agrees to listen to a demo of his hip-hop music. Affleck has now come clean. Virtually none of it was real, he told the New York Times. He acknowledged that Letterman was not in on the joke involving Phoenix who was nominated for an Oscar for his role of Johnny Cash in “Walk The Line.” Phoenix is due to return to “The Late Show with David Letterman” next week - and Affleck said this time he would not be in character.