Casey Affleck's much-hyped new documentary, 'I'm Still Here,' which follows a drugged-out, self-imploding Joaquin Phoenix as he tries to break into hip-hop, isn't real, Affleck admitted in a Thursday New York Times article.

"I never intended to trick anybody," Affleck, who's married to Phoenix's sister Summer, told the magazine. "The idea of a quote, hoax, unquote, never entered my mind." But, to the rest of the world watching, it appeared to be real. Phoenix played the role on- and off-screen for over two years--including an infamous appearance on the 'Late Show With Dave Letterman' in 2009 when he mumbled his way through the interview, leaving the host bewildered by the actor's behavior.

"It's a terrific performance, it's the performance of his career," Affleck says of Phoenix's two years of role-playing. He acknowledges that Phoenix put his professional life on the line to do this film--which Affleck reveals is modeled after the journalism of Hunter S. Thompson.