Casey Affleck: 'Joaquin Phoenix film is a hoax'

Casey Affleck has admitted to what many critics suspected all along - that his documentary about Joaquin Phoenix is a fake.

The film, I'm Still Here, followed Walk The Line actor Phoenix as he announced he was retiring from acting and launching a rap career.

The actor star grew doughy and dishevelled, and he famously made an awkward appearance on David Letterman's show.

Casey, who's Joaquin's brother-in-law and the film's director, has told The New York Times that the whole thing was an act, calling it "the performance of his career." Casey says he even faked home movies of Joaquin as a child.

But he insists he never meant to trick anyone, and rather was dabbling in "gonzo filmmaking" and describes Joaquin's performance as "compelling, always watchable, manages to be repulsive and charming, believable in all emotions, completely committed, incredibly brave."

Some critics called it a fascinating look at celebrity. Others assumed it was all staged.

Casey added that Letterman was not in on the joke.