"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," Phoenix said. "I hope I didn't offend you in any way."
No offense taken, Letterman replied. The chance to lob jokes at Phoenix was fun, he said.
Phoenix was clean-cut and shaven Wednesday, without the shaggy beard, unruly hair and sunglasses he wore on Feb. 11, 2009, when he told a befuddled Letterman that he was giving up acting for a rap career. Describing that appearance on Wednesday, Letterman said it was "like you slipped and hit your head in the tub."
Instead, Phoenix was playing a role for filmmaker friend Casey Affleck. The documentary on Phoenix's fake "career change," called "I'm Still Here," just hit theaters.
At Letterman's urging, Phoenix made clear that the talk-show host was not in on the joke.
"We'd hoped to come on a talk show," Phoenix said with a laugh, "and I was looking for a beat down, and I got one ... I want to thank you for that."
Affleck, who appeared with Jay Leno on the "Tonight" show Tuesday, said no one from Hollywood called to express concern about Phoenix when he was acting strangely.
"Afterward, the movie comes out, the critics like to say `this is crazy, this is disturbing, this is sick,"' he said. "But while it's happening, people were happy just to mock him and make fun of him."