Joaquin Phoenix has returned to "Late Show with David Letterman" and apologized for his surly, uncooperative February 2009 appearance.

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Casey Affleck recently admitted his brother-in-law Phoenix's bizarre behavior -- including the Letterman interview -- were part of a performance for their new film "I'm Still Here."

Initially billed as a documentary, the movie purported to be a chronicle of Phoenix's decision to quit acting and start a hip-hop career. During the past year, Phoenix has altered his appearance -- gaining weight, sporting a bushy beard and dark glasses in public.

Many in the entertainment industry have speculated the double Oscar nominee has been joking but the "Gladiator" and "Walk the Line" star hasn't rushed to explain his behavior.

"You're retiring to pursue a career in music. To me, that seems -- excuse me, I mean, you did sing in ('Walk the Line') -- but it seems unlikely because you've been nominated for two Academy Awards," Letterman said when Phoenix returned to the show Wednesday night.

"I, frankly, when I heard about it later, was surprised that anybody had believed it. And then after seeing you here -- first of all, really all I care about is me in this. Now, did I know anything about this (hoax?)"

Phoenix then responded, "No."

Letterman continued: "Yeah. Was there a script that you and I were working with?"

"No," Phoenix replied.

"Thank you very much," Letterman said. "I was not part of it, was I?"

"You've interviewed many, many people and I assumed that you would know the difference between a character and a real person, so -- but I apologize. I hope I didn't offend you in any way," Phoenix said.

"Oh, no -- oh, no, no, no. I was not offended. I'm telling you, it was so much fun. It was batting practice, you know what I mean? Every one of them was a dinger," Letterman quipped.

"Well, we'd hoped to come on a talk show," Phoenix laughed, "and I was looking for a beat down, and I got one. … I want to thank you for that."