'In Time's' Justin Timberlake Sings With David Letterman (Video)

Jeffrey R. Staab/CBS

Justin Timberlake broke into song on the Late Show With David Letterman Wednesday while promoting his new film In Time.

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Letterman asked the actor-singer if he'd ever be in a musical.

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"That's been discussed," said Timberlake, who added he's "not really" comfortable with the roles available.

"I would love to do a musical one day. I think there might be a way to redefine what that is for this era," said Timberlake. "My problem with musicals today -- and I think in my generation -- is that we're sitting her heaving a conversation and all of the sudden we're singing. Time stops."

Breaking into song, Timberlake pointed at Letterman's coffee mug and sang, "What is he drinking in that coffee mug?"

Pitched in Letterman, "La-la-la-hey-hey!"

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"This doesn't make it easy to get to know you," joked Timberlake, still in song.

Timberlake also admitted it's "wildly uncomfortable" to watch himself on the big screen.

But he was flattered "basically 40 friends" came to the Los Angeles premiere -- even if he was exhausted from promoting it and had to jump on a plane to New York City right after.

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"I felt like it was the first time they genuinely wanted to see me in a movie, and not Mila Kunis and Cameron Diaz… I sweated a lot… Not good stuff," he joked. "You're definitely critical of your performance, always. But I find the same with music, too. If a song of mine comes on the radio, which sometimes it still does, I find that I naturally want to change the channel. You pick it apart."