Joaquin Phoenix, Davis Letterman Joaquin Phoenix, Davis Letterman

Casey Affleck has admitted that Joaquin Phoenix's performance in the mockumentary I'm Still Here — including Phoenix's squirm-inducing appearance on The Late Show with David Letterman — was fake. But was Letterman in on the joke?

Casey Affleck: Joaquin Phoenix documentary is fake

"Dave knew about it and Dave loved it because he could play along," Bill Scheft, one of Letterman's writers, said in an interview with Nuvo magazine last year.

"I've told people that [everyone was in on the joke], and not only don't people believe me, they tell me that I'm wrong and that [Phoenix] is a schizophrenic and he needs help and he's going to end up like his brother [River]. I said no. I saw the segment notes. It's an act. I saw Ben Affleck's brother taping the whole thing from offstage."

Joaquin Phoenix returning to Letterman

With shaggy hair and a beard, Phoenix, 35, rarely gave more than one-sentence answers and appeared generally uncomfortable throughout the now-infamous interview. The appearance came shortly after Phoenix's announcement that he was giving up acting to become a rapper.

"It's a terrific performance, it's the performance of his career," Affleck told The New York Times of his brother-in-law's fictional portrayal of himself for I'm Still Here.

Still, Affleck contends that Letterman was in the dark. Perhaps all will be revealed when Phoenix returns to The Late Show on Wednesday, Sept. 22.