Feb. 12, 2009  -- A day after Joaquin Phoenix's bizarre appearance on David Letterman's "Late Show," the director of his final movie, "Two Lovers," said he's just as baffled about Phoenix's turn from Oscar-nominated actor to incomprehensible rapper as everyone else.

"If it is an act, it's the most committed act I've ever seen in my life," James Gray said in an interview with ABC News Radio's David Blaustein. "I mean, he built this studio [in his house]. The lengths to which he's taken it are quite extreme."

Phoenix's fatigue with acting was obvious while filming "Two Lovers," Gray said.

"Toward the end of the shoot, he kept saying 'Oh I'm so tired, I'm so tired.' You hear that kind of thing and you think it's a joke," he said. "I just ignored it."

So when Gray, who also worked with Phoenix on 2000's "The Yards" and 2007's "We Own the Night," found out via the Internet that Phoenix had vowed never to act again, he hunted down the star to confront him.

"I drove up to his house because his phone was disconnected," Gray said. "He said, 'I don't want to act anymore, I've been doing it for 30 years and if you did something for 30 years, you'd want to quit too.'"

Now, Gray wonders if he's responsible for Phoenix's decision to quit acting and take up rapping, a project he's pursuing with Sean "P. Diddy" Combs and Casey Affleck.

"Two Lovers," which also stars Gwyneth Paltrow and Vinessa Shaw, features an awkward freestyle rap performance by Phoenix -- not unlike his real-life rap debut. Gray revealed Phoenix was imitating him during that scene.

"That rap thing ... in the movie actually comes from something I played for him," Gray said. "I had an obsession with doing that sort of thing as a teenager. ... It turns out that Joaquin is imitating me in a lot of the movie. He said, 'I want to do that, I want to steal from that, I want to do the rap that you used to do.' I said, 'OK.'

"And now I'm seeing him do this thing, and I feel like I've ruined Joaquin Phoenix for the world," Gray added. "I don't want to be the guy that destroyed Joaquin Phoenix's acting career."

Phoenix Aloof, Out of It on 'Late Show'

Whether or not Phoenix is ruined as an actor, he's definitely out of sorts as a human being. In his interview with Letterman Wednesday, Phoenix, wearing a scraggly beard and dark sunglasses, seemed completely absent, staring off into space and answering questions with one-word answers.

Halfway through the interview, after a long pause from his guest, Letterman embraced the opportunity to make a fool of Phoenix.

"So what can you tell me about your days with the Unabomber?" he asked, eliciting howls of laughter from the audience. Phoenix didn't respond.

When Letterman ribbed Phoenix for chewing gum during the interview, Phoenix took the gum out of his mouth and stuck it to the "Late Show" host's desk.

That seemed to signal the end of the interview. Letterman closed the interview by saying, "Joaquin, I'm sorry you couldn't be here tonight."

Gray saw Phoenix Wednesday night, after the star taped his appearance on "The Late Show," but before it aired. Gray asked how the interview went.

"He said 'Oh it was good, it was really good," Gray said. "I watched it this morning ... I don't know what to say."