Fear not, all you Josh Groban fans — it’s a fat suit. When the popular singer steps onstage at the Imperial Theatre as the drunk and dour — and decidedly portly — Pierre, in the new musical “Natasha, Pierre and the Great Comet of 1812,” the first reaction of many showgoers is likely one of surprise. “Gee . . . he looks a lot heavier in person.”

But, no, he’s still hitting the gym. The padded costume is just one of the tools helping him get into character for this, his Broadway debut. In the raucous, immersive show (with book, music and lyrics by Dave Malloy), Groban is onstage much of the time, playing piano and accordion with the pit band, and observing as poor naive Natasha (Denée Benton) falls for the rock-star seductions (and crazy-high vocal range) of ne’er-do-well Anatole (Lucas Steele) — a love story plucked from the pages of Leo Tolstoy’s “War and Peace.”

A Los Angeles native, Groban, 35, has sold more than 30 million records, but his last appearance onstage in a musical was in high school (playing Tevye in “Fiddler on the Roof”).

Congratulations on your debut. It must’ve been nerve-wracking in the weeks leading up to your November opening. But I imagine the real test comes now, doing the show week after week.

Yeah. It’s interesting. It’s actually similar to a concert tour. When you’re on tour promoting an album, you often do the same concert all over the world. You change songs here and there but generally open and close with the same stuff. It’s great. I learn a lot from repetition. When you’re starting out, the nerves are there, you’re worried about critics, you’re amped up and doing it fully on adrenaline. So there’s something nice after you’ve done 50, 60, 70 in a row, when you start to discover nuances in the script — you find new things. And every audience is 100 percent different. It’s wonderful to get fresh faces every night.

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Fresh faces that are . . . in your face.

Yeah, if you’re in the audience, you’re gonna get spit on, that’s for sure. [He laughs.]

It’s amazing how they’ve redesigned the theater, so the action takes place throughout the audience. Is that tough? Your concerts are usually on a big stage with the audience at a distance.

At bigger venues, you’re usually looking out on an abyss — it’s dark out there, and you can’t see the audience very well. That’s why in my concerts I like going into the audience, pulling people onstage, doing anything to break that fourth wall. So I feel I fit right in here, with the audience all around — that’s when I feel most engaged. You can see everybody’s eyeballs. It keeps you honest as a performer, frankly. When everybody’s watching everybody else, there’s a feeling that the audience is part of it, so they’re more engaged than they normally might be, just sitting out there, ruffling through the program. It’s nice to have that energy.

It’s not distracting?

There are things that can be distracting. Somebody might reach out and grab you . . . and Josh would be delighted by that — “Heyyy . . . thanks.” Pierre? Not so much. So it’s fun to react to people in character. Pierre has no problem taking someone’s shaker away.

You mean those little percussion objects the audience shakes like maracas?

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He’ll take it away if they shake it too loudly. [He chuckles.] So the distractions are not really distracting. It’s just people being people. And people are unpredictable. But that unpredictability is what makes it fun for us.

You’ve changed your voice for this role. The lush baritone is still there, but Pierre is rougher, gruffer. Was it hard to find that sound?

It took a while, actually. Nothing’s more important to me than vocal health. I do pop music, but my training is primarily classical. Here, I needed to add fringe — add the sadness, anger, frustration and the brokenness of the character, while still maintaining vocal technique so I can perform eight shows a week. My voice is still my voice — I don’t want to change it so much that it’s unrecognizable. So it was a real dance . . . finding the right sound.

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You’ll be out of the show a few dates in the spring, when you have concerts lined up. What’ll that be like . . . being back onstage as yourself, singing in your own voice again?

Like getting back on a bike. I’ll have gained so much positive experience from this show and my fellow cast members. They inspire me every night. It’ll . . . be interesting . . . fresh. After you’ve done a thousand concerts, having something that maybe makes you scared again . . . is a good thing. A really good thing. So I think it’ll be a win-win.