He's practically made a career out of playing military men, but taking the role of a Roman centurion in Scotland nearly proved too much for Channing Tatum

• Channing Tatum in 'Fighting'

For a film actor he's surprisingly tall and broad, but that's not the only unexpected thing about Tatum, who laughs at himself a lot, and does his best to persuade me that his summer hit GI Joe was much worse than I thought ("the writers' strike meant that we had to rush it"). After spending time with "Chan" – who turns 30 this month – you may even be persuaded that he cares deeply about the music you are listening to – even though I own every David Bowie album going while he serenaded his director with Abba hits on the set of his last film.

Candid in ways that could cause Max Clifford to break out in hives, he chats easily about his Chippendale-esque days, when he was 18 years old and part of a troupe called Male Encounter. Rather try to sweep this under the carpet, he's hoping to make a movie based on his experiences with Bronson director Nicolas Winding Refn: "Not a comedy, though, I want to do the dark, ugly version. It was definitely one of the most interesting times in my life."

His exotic past doesn't bother him, it's just part of a time where he feels he paid his dues, sleeping in his car in New York while scouting for jobs, working in construction and as a perfume "spritzer" before walking into an agency and getting to flaunt his hotness as an underwear model. "It paid the bills, and it meant I didn't have to build houses or wait tables any more."

Up until now, he's been best known as the muscle in action flicks such as GI Joe and Fighting, or Step Up, a dance movie that became a box-office hit despite Tatum having never studied dance: "I just freestyled," he says. "I can usually grasp physical stuff pretty quickly but I'd never claim to be a dancer."

Now he's keen to flex different muscles with his upcoming movies The Eagle of the Ninth, shot in Scotland, Knockout, a Steven Soderbergh vengeance drama with Ewan McGregor, and romantic melodrama Dear John, out this month.

It's not his first romantic part – that would be the 2006 romcom She's the Man with Amanda Bynes – but it's a project he helped develop over three years, encouraged by his wife (and Step Up co-star) Jenna Dewan, who apparently adores anything by Nicolas Sparks, the novelist who also wrote The Notebook.

"I've seen the movie of it about 1,000 times, and it always makes my wife cry. And every time we reach the end she makes me promise we'll die in bed together, like they do," says Tatum. "And every time, I say to my wife, 'Can we please not talk about death again?'"

As a soldier who can only communicate with his lover through letters when he is posted to Afghanistan, Dear John puts Tatum in uniform once more, just as he was in GI Joe and Stop-Loss. "It's not that I worry about typecasting but the more I know about soldiers, the harder it gets to play them. I talk to them, I wear the clothes and I act out the tactical things that soldiers do, but I'm conscious that this doesn't mean I'm ever going to understand what it is to be a soldier."

To keep things relaxed on the set of Dear John, Tatum developed an easy rapport with Amanda Seyfried, who plays his love interest. "I felt very comfortable around Amanda. She's quirky and talented, and in a movie like this you need her sort of lightness. You can get too melodramatic, especially when you're doing love scenes with Lasse (Hallstrm, the director] shouting things like, 'Slower', 'Faster' and 'Don't move your heads so much'."

At least he shot his love scenes with Seyfried before he scalded his penis, an injury sustained on the shoot for The Eagle of the Ninth that was quite serious and not the least bit funny. (Oh, all right, maybe just a little bit funny.)

The Eagle of the Ninth is Scottish director Kevin Macdonald's first film since State of Play and is a 1st century AD drama about an idealistic Roman soldier investigating the disappearance of a legion in Britain. "My favourite movies are Braveheart and Gladiator, so I have been looking for one of these movies for a long time," says Tatum.

"All the Romans are American, which breaks away from the usual thing of making them English-accented villains. It's got a very contemporary theme and Kevin's clearly making an interesting statement about these Americans coming in and occupying a country."

I mention that when Mel Gibson shot Braveheart here, Scotland almost drove him mad, and Tatum nods enthusiastically. "For Fighting, I was doing bare-knuckle fighting and physically it almost broke me, but Eagle of the Ninth was the hardest things I've ever done. I have no idea how they are going to make the scenes match up because the weather constantly changes. When we did the wide shots there would be sunshine, but by the time we did my close-ups there would be torrential rain, and we'd shoot the reverse views in fog."

Co-starring Donald Sutherland as Tatum's warrior father, and Jamie Bell as his British guide, Scotland plays itself in the film, which was shot last winter around Wester Ross and Loch Lomond.

"I don't think I ever want to make another movie in Scotland," laughs Tatum. "I'd come back to Scotland again in a heartbeat because the Highlands are beautiful and the people are amazing but we almost died there a few times. Every single day I was freezing in a costume that was no more than a few thin strips of leather. I got mild hypothermia, Jamie Bell almost collapsed. And of course I did injure my penis."

This happened at the end of a long day where Tatum and his fellow actors were shooting a sequence where they waded across a river. Even with wetsuits on, the only way to stay warm was to have a mix of boiling water and river water poured down the suits. After the last take, one of the crew offered Tatum a final top-up – but forgot to add the river water. The scalding pain caused Tatum to pull the wetsuit instinctively away from his chest, which unfortunately sent the boiling water further down his suit, leaving "very little skin in places".

On the way to the hospital, Tatum begged his driver, an ex-forces marine, to knock him out. "I don't know if you've ever burnt your finger, but afterwards it keeps on burning for hours," he says. "To distract me in the car while I was being rushed to hospital, they switched on the radio – and it was the Kings of Leon singing Sex on Fire. Even I laughed at that, but it's the most suffocating pain I've ever felt."

The result sounds unimaginable – except Tatum took snaps of the damage with his phone. "I have a new iPhone now so I don't have the pictures to show you," he says, apologetically. "But I'm OK now, and I was back on set the next day. We didn't have any other option, so I just wrapped it up and carried on."

Despite a growing profile, Tatum continues to look for offbeat projects alongside blockbusters such as Captain America, "as long as people don't feel like, 'OK, you really need to do something different than a soldier.'" He's also hugely excited about Knockout, with director Steven Soderbergh. "It's an insane female Bourne-esque film and I get to play an assassin. I've always dreamed of working with him, so I can't wait."

Just as long as it doesn't involve wetsuits.