Joaquin Phoenix apologises to David Letterman

The "mockumentary" was premiered at the Venice Film Festival

Actor Joaquin Phoenix returned to David Letterman's Late Show on Wednesday to apologise for his strange behaviour when he last appeared as a guest.

The star seemed confused and incoherent during the interview in 2009, but it has since been revealed that it was part of an act for a mock documentary.

Phoenix, who insisted the US host had not been in on the joke, said: "I hope I didn't offend you in any way."

I'm Still Here is about Phoenix quitting acting to become a rap star.

Last week actor Casey Affleck, who made the film, admitted that it had been staged.

Over the last two years, Phoenix has behaved strangely in public, leading fans and critics to wonder whether he was documenting a breakdown on film.

Speaking to Letterman, he said: "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."

Unlike his appearance last year when he turned up with a long beard and unkempt hair, Phoenix was clean-cut and shaven.

Referring to the interview, Letterman said it was "like you slipped and hit your head in the tub".

"I was eager to thank you for the opportunity and I was really eager to get off the show," Phoenix said.

The film, which was premiered at the Venice Film Festival, was released earlier this month in the US.