I was a stuntman with Leonardo DiCaprio and now I'm singing the lead at The Royal Opera

Yup, that tall bloke with the buzz cut and striped shirt standing behind Stephen Graham in Leonardo DiCaprio's posse from Gangs of New York is Italian tenor superstar Riccardo Massi. He's also pretty handy with a double sword, spear and dagger. Just who you'd want on your side in a bar fight (real or Hollywood).

No wonder that when he turned up in Milan in 2007 to audition for La Scala's Young Artist opera programme with a shaved head, swollen eye and covered in cuts and bruises, it did not, initially, go well.

"They wouldn't let me in," Riccardo laughs. Not quite the hooligan he appeared, he'd been funding his music studies for years as a nightclub bouncer and as a stuntman at Rome's legendary Cinecittà studios, where he'd also been able to observe Hollywood legends at work.

Although, he tells me that the one who impressed him the most was not Leonardo DiCaprio...

We're meeting to promote his starring role in Il Trovatore at the Royal Opera House. It's a very long way from his stuntman days and his upbringing far from the artistic worlds of Rome and London.

"I do not come from a family of artists," he says. "My father was an entrepreneur who loved opera but can not sing. My mother owns a shoe shop and never sang, but she has perfect pitch. So, I am a strange mix of them."

Growing up in tiny Sarnano, a remote central Italian town of a few thousand, with not much to do, he studied karate at the local gym. After high school, he moved to Rome and trained in Brazilian jiu-jitsu and Muay Thai. "I joined a dojo that was teaching medieval fighting with the sword and shield, spear, knife, double sword," he says. "It was very well-connected with the stuntman community."

For almost a decade, he worked on local and international projects, learning from all the stars: "We were not allowed to speak to the actors," he says, "but I was doing what Italians call 'steal with your eyes.' Especially Daniel Day Lewis in Gangs of New York. That guy is amazing, always in character, even off camera."

Few could imagine that his biggest dream was to sing: "It was there in me," he says. "I said, 'okay, let's try,' and I found a teacher."

And then one day, after filming a brutal, bruising fight scene for the HBO series Rome, he recalls that the phone rang: "I'm wasted, I just want to go home and take a shower," he laughs, "and La Scala wants to see me in two days! I tried covering it all very badly with make-up, but it didn't work!"

Riccardo eventually convinced the dubious doorman to let him pass and aced the audition, making his major debut just two years later in 2009 as Radamès in Aida at the Teatro Verdi of Salerno. The Egyptian general torn between fighting the Ethiopian armies and loving their princess was tailor-made for him.

"It definitely goes with my personality," he grins. "If you ask me if I like more to sing Radamès or Rodolfo in La Bohème, they're both beautiful, the music is made in heaven, but I feel much more like the young guy that that leads an army and makes unbelievable sacrifices than the young poet with no more struggles than love, which is beautiful, but when it's just only that, I feel that something is missing, at least for me."

He still uses his old skills in fight scenes but some of the muscles had to go. Looser, supple bodies suit the mechanics of opera, he explains: "You don't have to be a couch potato, but you cannot be a gym rat. You also need endurance to sing kneeling or lying on the ground or on steps.

"Sometimes people forget this, but I am my instrument. If I sleep or eat badly, it affects my voice. The discipline is the same as martial arts. Eat well, do not yell, drink or smoke. I swim to stay fit with lots of stretching."

He also busts a major myth that larger bodies create a richer, fuller voice: "The sound an opera singer produces goes out of your body through the vibrations of your bones and your skull. 30 or 40 extra kilos doesn't affect the way you sing. But it can affect your health!"

Not all voices or operas are the same, though. I tell him I love the pretty Italian and French pieces, but give me something German and I'm in trouble. Anything Russian, I'm out the door.

"Yeah, me too" he laughs. "I am a dramatic tenor. That covers Bel Canto, the period of Donizetti, Verdi until we arrive to Puccini."

Does that mean he is a drama queen? "Some of them are," he raises his eyebrows, "but not me! It means that you have a voice that is capable of that repertoire. Not just the volume, but also the colour and the timbre. Your voice chooses your repertoire, not you. I am very happy because I love the romance and structure of the music. Wagner requires a type of singing that's a little bit more violent. If you don't know your limits as a singer, you can hurt your vocal cords."

You also have to come out and tackle numbers like Nessun Dorma under the shadow of the all-time greats like Pavarotti or Franco Corelli.

"Everybody's waiting for you," he laughs, "and it's like (he puts on a squeaky voice), 'I'll do my best. I'm sorry.' Nobody can beat them, but they're gone."

Opera audiences, of course, are notorious for heckling and booing. "They do!" he grins. "It happened to me. It's part of the game. It's not necessarily, 'you suck.' It's, 'we wanted another singer' or they don't like the production. They get very angry. You can give your soul, everything you have, but you will never ever be able to please everybody."

Often lazily labelled as elitist and old-fashioned, opera struggles with modernising its appeal while pleasing those dedicated traditional supporters.

"Opera is eternal," he says. "Thank God. Why? Like Shakespeare, it speaks to human emotions, which have been the same for millennia. Aida is basically about two kids loving each other and another girl trying to get between them. In Il Trovatore, it's two men vying for the same woman. And when the alchemy is magical, pure emotion fills you and it is so, so beautiful. But it will need to evolve, adapt to the new times.

"Damn, this is a very delicate subject," Riccardo pauses, aware of the looming minefield...

"Traditional staging will always work because the composer thought about it in that way," he says. "And I actually know many young people are coming and they love traditional stagings. With modern productions, you risk doing something that people will not believe. If you set Aida in a gypsy camp or a degraded suburb, you gotta have a very, very good idea to make me forget that we're not in Egypt with pyramids and deserts. If you do not involve me in your vision, I will get bored.

"If you betray the original spirit of the piece, everything will fall down And this is terrible because you failed. The primary objective is to bring me into another world for three hours. Unfortunately, if you ask me, one out of 30 modern productions work. It's painful for an opera singer when you look at the first rows and people are looking at their watches."

It's a joy to talk to an artist who sees beyond his art to the other side of the equation, to the audience. But I'm still blown away when I ask what he enjoys away from his work, cheekily imagining afternoons reclining on a chaise in a silk kimono, and he proudly declares he's a fellow "sci-fi and fantasy nerd."

We share our pain about people, even friends, not understanding that, like opera, it's a way of looking at human interactions, what is happening in the world today, through a lens. We geek out about the upcoming third season of Isaac Asimov's Foundation on Apple TV and get way too sidetracked debating Disney's patchy run of Star Wars shows.

"I grew up with Star Trek and Star Wars, it gave me life," he beams, "Andor was a masterpiece but I didn't much like Kenobi with Ewan McGregor. It was a wasted chance."

And then he finishes me off by flashing his engagement ring (to fellow singer Elisa) and I see a gleaming band wrapped with Elvish script from Lord of the Rings.

May opera, ahem, "live long and prosper" with people like Riccardo at its beating heart.

Riccardo Massi stars inIl Trovatore at The Royal Opera House from July 8 (rbo.org.uk)