Joaquin Phoenix’s appearance on the David Letterman Show Wednesday night left fans wondering whether or not the two-time Oscar nominee had completely lost his mind.

After perplexing fans recently by announcing he was quitting acting to become a hip-hop artist, the 34-year-old appeared on the talk show to promote his last film, Two Lovers, but gave only stammering one-word answers.

At one point, he pulled a wad of chewing gum from his mouth and stuck it to the host’s desk.

Letterman welcomed him with a glowing introduction and initially worked hard to coax answers out of the unresponsive star. But after receiving a raft of one-word replies, the presenter, known for his cutting wit, gave in to the live audience’s disbelieving laughter and began mocking the unkempt-looking Phoenix.

Taking aim at his shaggy beard, he enquired: “What can you tell us about your days with the Unabomber?”

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Phoenix appeared mildly offended, but continued on with the interview, later informing Letterman that he was putting acting behind him in favor of his music career as a rapper.

Still, what fans did not see was the presence of a film crew, led by the Walk the Line star’s brother-in-law, Casey Affleck. The film crew, coupled with Phoenix’s train wreck debut as a hip-hop artist, continue to raise the question … Is Joaquin headed down Amy-Winehouse-lane, or are the crazy antics and shaggy appearance all part of an elaborate joke?

Phoenix has vehemently denied that the career-change is a stunt.

"There's not a hoax," Phoenix said. "Might I be ridiculous? Might my career in music be laughable? Yeah, that's possible, but that's certainly not my intention."

Still, despite his intentions, his music indeed was laughable, at least to Letterman’s audience, who could barely contain themselves when Phoenix spoke of his ambitions as a rapper.

Seemingly annoyed by the audience’s response, he asked Letterman, “What do you have them on? What do you gas them up with?”

The same could certainly be asked of Phoenix himself.

VIDEO: Click here to see the full interview.

The Associated Press contributed to this report.