Paul Mescal's “Gladiator II” trainer had him strip down to see how fit he was: 'Into your underwear, please'

His diet? "All of the chicken imaginable."

Paul Mescal's buffed up look in the new Gladiator II began with him stripping down.

"He comes to the hotel and he's like, 'Can we go up to your room?'" Mescal said Wednesday in an appearance on The Late Show with Stephen Colbert. "I'm like, 'Yeah, that sounds good.' And he goes, 'Into your underwear, please.' And I'm like, 'This is how this works. OK.' So I'm like [standing] there in my underwear, and he's just kind of like circling me and, like, doing his, like, mental... basically, just doing, like, looking at my body scientifically."

See his interview in the video below:

Mescal said there was then intense silence.

"And he's just looking at me with his head cocked," the actor said, "and he goes, 'There's a canvas with which to work here.'"

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Mescal laughed while telling the story, but the trainer was serious about giving him numbers of how much weight he wanted him to add to each body part.

The 28-year-old actor said his diet was "all of the chicken imaginable."

He and the trainer worked together for "two or three months leading up to it," Mescal said. "I was on stage at the time, which was really useful because you get days to yourself."

Related: Now entering the arena: Paul Mescal and Pedro Pascal unsheathe Gladiator II

Mescal recently told Entertainment Weekly that the film, the sequel to Ridley Scott's 2000 best picture winner at the Oscar that starred Russell Crowe, was the most physically demanding of his career. He plays Lucius, who's captured from the place he'd been sent for protection and taken back to Rome, where violence ensues.

"I loved how distinct the fights are from each other, and what I'm really proud of is how you can feel the cumulation of the violence on his body as the film progresses. The fights aren't like slick swordplay," he said. "You can feel towards the end what Lucius says — it's about survival. It's like your body's going to accumulate all this punishment. And it's about holding onto that as the film progresses. And I think it's much more interesting to see somebody who psychologically has an aptitude for survival rather than somebody who's innately skilled. And I think Lucius just has a dog in him that will perpetually survive environments that other people shouldn't. So I'm proud of them all in different ways."

Related: Denzel Washington reveals he kissed a man on the lips in Gladiator II but it was cut: 'They weren't ready for that yet'

Mescal noted on Colbert that he's no longer in the same place physically as he was during filming.

"Oh, how the mighty have fallen," he joked. "This is not what it's like here at the moment, unfortunately."