Joaquin Phoenix apologized to David Letterman on 'The Late Show' (weeknights, 11:35PM ET on CBS) for his previous bizarre appearance on the show back in February 2009.

That interview was a memorable event for all concerned.

Back then, Joaquin sported a long beard, unruly hair and dark sunglasses. He muttered his way through his 10-minute segment on the show, barely answered any questions, and then bizarrely announced that he was quitting acting to start a new career as a rapper.

Well, since that last interview, Joaquin has released the "mockumentary" 'I'm Still Here,' which he shot along with his friend Casey Affleck. The movie purports to cover his "lost period" -- and features full-frontal male nudity, topless call girls, tons of drugs, and a now-infamous scene involving defecation. However, following the movie's release, director Affleck revealed that the entire thing was a hoax.

Tonight, Joaquin was back on 'Letterman,' to explain what in the world he was thinking. Let's hear what he has to say for himself, shall we?

First of all, this time around, Joaquin was clean-shaven, talkative and upbeat. He told Letterman that the purpose of his previous appearance was to provide something "authentic" for the mockumentary. He apologized for his behavior, and said that he hoped he hadn't caused any trouble.

Joaquin also explained that his movie project was inspired by reality TV. "I'd started watching a lot of reality shows," he said, "and, um, I was amazed that people believed them -- that they called them, like, 'reality.' I thought the only reason why is because it's billed as being 'real,' and the people use their real names. ... I thought I could handle that. ... You just use your name, and people think that it's real."

This was the actor's explanation of his entire 'I'm Still Here' acting project. So, in other words, Joaquin was attempting a performance art piece that critiqued the nature of "reality" itself. He wanted to act like a caricature of himself, and see if people would fall for it.

So there you go! That's the explanation! And now the hoax is over, and everything can go back to normal. ... Or maybe not. Because on tonight's show, Letterman posed this question to Joaquin about the 2009 interview: "Did I know anything about this?"

"No," was the answer. That is to say: Both Letterman and Joaquin were declaring that last time around, the host had no knowledge about the hoax, and thought that the actor really was crazy.

Hmmm ... That's interesting, because this week, Casey Affleck admitted that he told Letterman's sidekick Paul Shaffer about the prank beforehand -- making it incredibly likely that Letterman knew about it as well. And as if that wasn't enough, one of Letterman's writers, Bill Scheft, recently confirmed that the host knew that Joaquin was acting: "Dave knew about it, and Dave loved it because he could play along."

So it seems like the prank is still continuing, sort of. Letterman's acting like he believed that Joaquin had gone crazy. But really, he was in on the whole deal, as far as we can tell.

What do you guys think? Is Letterman lying? Is Joaquin still lying? How long can this hoax go on for? Will the strangeness ever end? And did you find Joaquin's weirdness entertaining, or did you think that this whole stunt just fell flat?

Check out the entire interview below: