Casey Affleck has finally admitted – more or less – that I'm Still Here, his documentary on the apparent decline of actor Joaquin Phoenix, is an elaborate hoax.

“It’s a terrific performance, the performance of his career,” Affleck said in an interview with the

New York Times.

He went on to confirm that Phoenix’s appearance on David Letterman’s show, during which the absurdly bearded star mumbled incoherently, was a performance piece, but noted that Letterman (who seemed visibly irritated) was not in on the gag.

I’m Still Here

, which opens in the Irish Film Institute next week, follows the Oscar-nominated Phoenix as he gives up acting to begin a career as an appallingly inept rapper.