It’s tough being a poster boy for opera. The American baritone Nathan Gunn has inspired a Facebook group, unashamedly called “I want to touch his pecs”. When he appeared on Comedy Central’s Colbert Report he was introduced as “Nathan Gunn, super-sexy opera star”, flashing up images of a bare-chested Gunn in Tobias Picker’s opera An American Tragedy.“ Do you ever perform with your clothes on?” Stephen Colbert quipped.

What’s an opera star to do to be taken seriously? For his first major operatic appearance in the UK, Gunn has chosen a decidedly unhunky part in Peter Eötvös’s new opera, Love and Other Demons. Based on the Gabriel GarcÍa Marquéz novel, it premieres tomorrow. In brief, it’s The Thorn Birds with exorcism. Gunn plays Father Delaura, who is tortured by his love for a 12-year-old girl, Sierva MarÍa (sung by the soprano Allison Bell). Sierva has been bitten by a rabid dog, and the superstitious townsfolk believe that she is possessed. Delaura is dispatched to exorcise her demons, but the pair fall in love. Inevitably, they are discovered and “everything goes to hell”.

“It’s pretty wild,” Gunn adds laconically, munching a salad in the Glyndebourne restaurant. Rehearsals are tiring, with tough technical demands on the set. After lunch Gunn will film a bloody self-flagellation. There are a few of these mini-films, representing the thoughts and dreams of the protagonists, which will be projected as the action unfolds on stage.

With his cool glasses, a goatee beard and those muscles discreetly hidden under a black T-shirt, Gunn will have to go some to become the nerdy priest he describes. “Delaura is comfortable in his vocation. He has a love of books, his world is peaceful and organised. Then he is asked to exorcise this girl and whatever she has – goodness, innocence – attracts him.”

Marquéz’s 18th-century Colombia is a fantastical world, yet Gunn identifies something of himself in Delaura. “All of us think we’ve arrived at where we’re going, and suddenly we realise that’s not the end of the journey,” he says. In Delaura’s case, the priest believes he has reached his goal – the Church – only for life to throw a spanner in the works. “I thought I was going to have three kids and then my wife got pregnant with twins, so now we have five,” Gunn says. “All of a sudden life changes.” Even his career was fortuitous. One summer, needing extra cash, he took a job singing at weddings. His mum found him a singing teacher, who played him his first opera, Mozart’s The Magic Flute. “And I thought: ‘Wow! This is absolutely beautiful. Maybe I should audition for music school instead of going to the air force academy’. ”

Gunn got into every music school to which he applied, ending up at Illinois before honing his craft on the New York Metropolitan Opera’s young artists scheme. We haven’t seen much of Gunn in the UK. He sang a concert version of Billy Budd with the LSO last year, he has appeared in Ariadne auf Naxos at Covent Garden, and in 2006 he played the astronaut Buzz Aldrin in Man on the Moon, a TV opera written by Jonathan Dove for Channel 4. It’s one of five new operas on his CV, including André Previn’s take on Brief Encounter for Houston Grand Opera next year. Gunn has an appetite for new work and, with composers beating a path to his door, the story is the first thing that he considers.

“I think about text a lot – I’m not a ‘put it in your snout and shout’ singer,” he says, relishing the opportunity to create a role. “It’s so nice to engage an audience in a fresh interpretation of a story and not to match up to the best performances they’ve seen.”

Love and Other Demons won him over on two counts: its intriguing story and the chance to work with the Hungarian composer Eötvös. For his first UK opera commission, Eötvös has tried to create a new relationship between singers and instruments. The orchestra is split in two, with a third block of players in between, providing counterpoint and overlapping musical colour. “It’s dramatically very effective,” admits Gunn, who received the completed second act only in April.

Since Eötvös uses the voice for effect, the pair met last year to talk vocal range. Whatever they discussed, the composer has Gunn yelling, whispering, even singing falsetto and frequently crooning one thing while the orchestra plays another.

Gunn realises that he is starring in the hardest sell of Glyndebourne’s summer festival. “At least to standard opera-goers,” he suggests, arguing that younger punters will be more open to the experience. “New pieces put young people on a level playing field with those who’ve been going to opera for years; nobody knows what to expect. And the way pieces are written now, the drama is more critical – it’s woven into the music.

“If I were in the audience I would listen closely to the text and let that take me into the music. It’s like a movie; you’re really listening to the dialogue but if you take the music out, it would totally change your perspective of what’s happening.”

Gunn makes a fine, articulate ambassador for all types of music, from Benjamin Britten to Broadway musicals. His last role was singing Gaylord Ravenal in Showboat and his current album is a slick collection of romantic tunes. If opera colleagues are sometimes baffled, Gunn has no problem existing in the two camps. At high school he was the sports jock – on the American football team – who also sang in the choir. How very unGrease. “To serve the music is to serve the music,” he says, simply.

The role of Father Delaura, however, will undoubtedly define him in British eyes as a serious performer. Gunn hopes not. “It bothers me that singing [opera] and the performing arts are so separate. When I do pieces such as Love and Other Demons,it allows me to do Camelot or Carousel. And to do all of them gives credence to the validity of all these different styles of music.”

Finally I wonder, what does he think defines him among today’s top baritones? It can’t be the buffed bod – Christopher Maltman has that. Nor the athletic performances – Simon Keenlyside can match those. After a thoughtful pause, Gunn gives an answer that should have Glyndebourne rubbing their hands: “I try to make contemporary pieces not sound contemporary. I want to engage the modern public in an art form that deserves to last a long time.”

Love and Other Demons, Glyndebourne, Lewes (www.glyndebourne.com 01273 815000), from Sun. Some £30 tickets are available for under 30s

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