SOUTH PASADENA, Calif.

CASEY AFFLECK wants to come clean.

His new movie, “I’m Still Here,” was performance. Almost every bit of it. Including Joaquin Phoenix’s disturbing appearance on David Letterman’s late-night show in 2009, Mr. Affleck said in a candid interview at a cafe here on Thursday morning.

“It’s a terrific performance, it’s the performance of his career,” Mr. Affleck said. He was speaking of Mr. Phoenix’s two-year portrayal of himself  on screen and off  as a bearded, drug-addled aspiring rap star, who, as Mr. Affleck tells it, put his professional life on the line to star in a bit of “gonzo filmmaking” modeled on the reality-bending journalism of Hunter S. Thompson.

“I’m Still Here” was released last week by Magnolia Pictures to scathing reviews by a number of critics, including Roger Ebert, who wrote that the film was “a sad and painful documentary that serves little useful purpose other than to pound another nail into the coffin.”

“The reviews were so angry,” said Mr. Affleck, who attributed much of the hostility to his own long silence about a film that left more than a few viewers wondering what was real  The drugs? The hookers? The childhood home-movie sequences in the beginning?  and what was not.