Letterman knew Joaquin Phoenix was faking

Host 'loved it because he could play along,' says writer

David Letterman was in on the joke.

Despite Casey Affleck claiming nobody knew Joaquin Phoenix was faking his bizarre behavior for "I'm Still Here" (except for his agent), one of Letterman's writers, Bill Scheft, said in an interview last year: "Dave knew about it and Dave loved it because he could play along."

"It was great television," added Scheft to the independent newspaper, Nuvo, in an interview that was dug up Friday in light of Affleck's admission Thursday. "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." (The actress rambled awkwardly during a 1997 appearance.)

Scheft said he was rebuffed when he tried to expose Phoenix.  "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, who died of a drug overdose in 1993]. I said no. I saw the segment notes. It's an act. I saw Ben Affleck's brother taping the whole thing from offstage."

Affleck admitted to the New York Times Thursday that Phoenix's behavior was staged: "It's a terrific performance, the performance of his career.”

Phoenix will return to Letterman's couch next week.

As The Hollywood Reporter has written, the aberrant behavior hasn't hurt Phoenix's career; he's weighing his next film roles.