Actor-turned-rapper Joaquin Phoenix was left squirming on U.S. TV after talk show host David Letterman grilled him about his new furry beard and burgeoning hip-hop career.

The star hid behind dark sunglasses and chewed gum as he promoted his movie Two Lovers - the last project he filmed before announcing his retirement from Hollywood - on Letterman's Late Show on Wednesday.

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Letterman asked the star about his new unkempt facial hair but Phoenix was uncooperative, mumbling one-word answers.

The former actor then discussed his new musical career but grew visibly annoyed when the audience began to laugh, grumbling: "Is this a joke? What (drug) do you have them on? What do you gas them up with?"

And when the house band leader Paul Shaffer laughed at the pair's awkward exchange, Phoenix turned to him and asked: "Are you f---ing kidding? Are you serious with the maniacal laughter?"

Letterman tried to assure Phoenix, insisting: "We're having fun, just relax, seriously! I'll come to your house and chew gum!" - prompting the star to spit out the wad and stick it onto Letterman's desk.

The host concluded the interview by saying: "Joaquin, I'm sorry you couldn't be here tonight."

(This news article provided by World Entertainment News Network)