The Oscar-nominated actor made an incoherent guest appearance on Letterman's Late Show last year to promote Two Lovers, in which he wore sunglasses, chewed gum and refused to answer the host's questions.
However, in an interview conducted last summer for Nuvo, Late Show writer Bill Scheft said that Letterman was part of the hoax, and compared Phoenix's performance on the show to late comic Andy Kaufman.
"Dave knew about it and Dave loved it because he could play along," Scheft said. "He could do whatever he wanted with it. And he did, and it was great television. But I will take credit for the line, 'I think I owe Farrah Fawcett an apology'. That line was mine. I gave that to him during the break."
He continued: "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. I said no. I saw the segment notes. It's an act. I saw Ben Affleck’s brother taping the whole thing from offstage."
Phoenix will return to The Late Show on Wednesday night.