Joaquin Phoenix publicly apologised to US talkshow host David Letterman on Wednesday when he returned to America's Late Show and revealed all about his odd antics on the programme last year.

The Walk The Line star appeared dishevelled and uncomfortable as he attempted to promote what he claimed would be his final film, Two Lovers, on the show in 2009.

He evoked the presenter's wrath by refusing to remove his sunglasses for the studio interview and chewing gum as he spoke.

Phoenix's brother-in-law, director Casey Affleck, has since admitted the appearance was part of an elaborate "social experiment" stunt he had been filming for his new documentary I'm Still Here, about the actor's spoof efforts to become a rapper.

Appearing on the show for the first time since the incident, Phoenix told Letterman that he was sorry for his behaviour, which was since confirmed to be an act for his mock documentary I'm Still Here (2010).

Phoenix told the host on Wednesday's Late Show: "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 - but I apologise. I hope I didn't offend you in any way."

"Oh no. Oh no, no, no," Letterman replied. "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."

The comedian also denied suggestions that he was in on the hoax.

"Was there a script that you and I were working with?" he asked Phoenix, to which the actor responded: "No. We'd hoped to come on a talkshow, and I was looking for a beat down, and I got one... I want to thank you for that."

Source: Press Release