It's been over a year since Joaquin Phoenix appeared on 'The Late Show with David Letterman' sporting an unkempt beard and sunglasses and mumbling his way through an awkward, often-uncomfortable interview with the late-night host. And on Wednesday, a clean-shaven Phoenix returned to the show and apologized.

It turns out it was all an act--he was playing a character for his friend Casey Affleck's faux documentary 'I'm Still Here.' Affleck revealed last week that the movie, and Phoenix's character, was fake.

"I mean, I think that you've interviewed many, many people and I assumed that you would know the difference between a character and a real person," the actor said to the late-night host. "I hope I didn't offend you in any way."

"Oh, no. Oh, no, no, no," Letterman said. "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. I was looking for a beat down, and I got one... I want to thank you for that."