'This Is Us' guest star breaks down his nerve-racking singing debut with Mandy Moore

Sam Trammell makes his singing debut on Tuesday's episode of "This Is Us."

"I got to actually sing with Mandy Moore," Trammell told Business Insider on Monday. "I've never sung before."

Trammell, who starred as Sam Merlotte on the HBO hit "True Blood" for seven seasons, is currently appearing in a guest role on the NBC drama as Ben, an old friend of Moore's Rebecca from when she was pursuing a singing career.

Ben convinces her to get back onstage and sing with his band. At the same time, he potentially presents some challenges to her relationship with husband Jack (Milo Ventimiglia).

"We go on tour and that creates some drama between us two," Trammell teased.

While he's never sung professionally, Trammell is an accomplished musician. He plays a variety of instruments. Aside from being in rock bands from the age of 13 through his colleges years, Trammell plays the classical piano, was a member of a youth symphony, and loves the cello. That doesn't mean he jumped at the chance to sing when "This Is Us" first offered it to him.

"They asked me if I could sing and I said, 'No, I don't sing, but I play all these instruments,' I told my agent," the actor said. "['This Is Us' executive producer] Ken Olin called me and asked if I wanted to try it, so I said, 'Yeah, I definitely want to try it.' I should have said that in the first place."

Like a true professional, Trammell hired a singing teacher to prepare for the song and worked closely with the composer.

"We got the key down, so it would be in my voice register, so I can sing it," he said.

But once he and Moore hit the recording booth, the platinum-selling pop artist threw her duet partner a couple curveballs.

"Funny enough, Mandy wanted to sing it higher, because she could really rip with the key higher," Trammell said. "So the composer said, 'Sure, we'll do that.' And then he looked at me and said, 'Don't worry, Sam, we'll figure it out.' I was like, 'Oh, my God.'"

The actor said he was "blown away" by Moore's singing on the song, which made him nervous about his turn in the booth. Then again, the actress threw him a curveball.

"She came out [of the recording booth] and I said, 'Good job. You can go. I'm just going to be in here,'" Trammell said. "She said, 'No, I'm going to sit around and watch you, give you some support.' I was like, 'Oh my God.' But it totally worked out. My teacher came through for me and we got to sing in the episode and it was a blast. Obviously, I had such a fun time doing it."