Joaquin Phoenix

Better call it a mockumentary.

Joaquin Phoenix's recent bizarre behavior, chronicled in the supposed documentary, I'm Still Here, was all a ruse for the film, director Casey Affleck says.

Check out photos of Joaquin Phoenix

"It's a terrific performance, it's the performance of his career," Affleck tells The New York Times of his brother-in-law's fictional portrayal of himself, which included his baffling interview with David Letterman last year.

During the two-year production, Phoenix left many scratching their heads as he "retired" from acting to launch a rap career while adopting a bearded, disheveled look, and a hard-partying lifestyle.

But it was all an act, as the two wanted to dabble in "gonzo filmmaking."

"I never intended to trick anybody. The idea of a 'hoax' never entered my mind," Affleck says, adding that he wanted people to watch the film without any preconceived notions. "We wanted to create a space. You believe what's happening is real."

Joaquin Phoenix returning to Letterman

Affleck says Letterman wasn't in on the joke when Phoenix dropped by in February 2009 for an awkward interview, which the host famously punctuated with, "Joaquin, I'm sorry you couldn't be here tonight."

Phoenix is scheduled to visit Letterman on Wednesday.

Affleck, who also faked childhood home movies of Phoenix, says one part of the film wasn't staged: footage of Phoenix and his siblings performing on the streets of Los Angeles like the Jackson Five.