Filmmaker Casey Affleck, whose film "I'm Still Here" is about Phoenix's apparently aberrant behavior, finally confirmed it once and for all to be performance art in an interview this week with the New York Times.

“It’s a terrific performance, it’s the performance of his career,” Affleck told the Times of Phoenix’s two-year portrayal of himself — on and off camera — as a drugged-out, bedraggled and bearded would-be rapper.

At the time of the Feb. 2009 TV interview, some saw it as a committed performance in the style of the late Andy Kaufman. Others simply thought Phoenix should be committed.

Letterman, Affleck said, was not in on the hoax but did what he could to salvage the interview, which should make for some interesting conversation when Phoenix returns to CBS' "Late Show' next Wednesday night.