I’m coming out of the London Coliseum, home to the English National Opera, when a flyer is thrust into my hand bearing the brooding face of Bryn Terfel, with the words “Bryn Terfel IS Sweeney Todd, the Demon Barber of Fleet Street.” The black and white photograph of him, unshaven and menacing, carries the message: Be Afraid. Be Very Afraid. “I’ve just interviewed him,” I say to the flyer woman. “Have you?” she says excitedly. “Doesn’t he look gorgeous?”

Bryn Terfel is rather gorgeous. However, he has never had any knickers thrown at him. “No, no, no, not as far as that,” he says. “I’ve seen Tom Jones sing three or four times in Vegas, and knickers are being thrown at him – but mostly by Welsh ladies.”

Fortunately for Terfel, the rarefied world of opera does not do knicker throwing. Not that Terfel would see himself as a rarefied object. Quite the opposite. The world’s greatest living bass-baritone is astonishingly charming and down-to-earth. He opens the car door for me, pours the tea, and even dismisses the taxi to give me a lift back to the station.

He does, however, have one big “ask”. That you come to meet him in North Wales, near his home town of Pantglas, where he lives with his wife Lesley and three young boys. This is not just out of convenience for him, but also because he believes that the joy of Wales should be shared. While he says he has no political ambitions, and is not a Welsh nationalist, Terfel does see himself as an “ambassador to Wales through music”.

We meet in a hotel near his home, which is supposedly haunted. When Angelina Jolie and Martine McCutcheon came down for Terfel’s annual music festival, they refused to go into our interview room “because of all the spooks flying around. They had to ask Russell Grant to try and get rid of them,” says Terfel.

It seems fitting to talk of the dead before we discuss Terfel’s UK debut as Sweeney Todd, the Demon Barber of Fleet Street at the newly refurbished Royal Festival Hall. Stephen Sondheim’s 1979 masterpiece is dubbed a musical thriller, but has the form and construct of an opera, and the story is dark with elements of comedy.

Some opera snobs are sniffy about Sondheim, but Terfel sees him as a “genius”. “You don’t throw the term ‘genius’ around very often but this guy is one.” Terfel had the opportunity to meet Sondheim when he performed the title role in Chicago, and was awestruck by the occasion. “He had to share his time with everybody, but my ten minutes with him was one of the most amazing ten minutes I’ve ever spent in a dressing room in an opera house,” says Terfel.

Sondheim had written some notes about his performance, but threw the comments in the bin. “I didn’t just want to hear the positives, I said that I wanted him to tell me everything – the good and the bad,” says Terfel, who admitted fishing the notes out of the bin to flatten down carefully in his score. “At the end of the soliloquy before the duet with Mrs Lovett [who will be played by Maria Friedman at the RFH], he wanted me to hold on to the declamatory note as long as I possibly could, and I tried every night to do that.”

The story is based on a 19th-century tale. Todd returns home from the penal colonies in Australia where he has spent 15 years on false charges. He comes back to find his wife has poisoned herself after being raped by Judge Turpin, the judge who imprisoned him, and his daughter in the care of the judge as a ward.

To seek revenge, Todd enlists the help of Mrs Lovett, “the worst baker of pies in London”, as a co-conspirator, and the result is mass murder, where victims are baked into pies. Referring to Opera North’s production some years ago, Terfel remembers “coming back in to the second half and smelling the pies in the air. They’d put some in the microwave. Your first thought was, ‘What a lovely smell of pies’ and then you remembered what was in them.”

Before coming to the Royal Festival Hall “to do a psychopathic murderer that kills at will”, Terfel visited Salt Lake City in America, where he had been singing Mendelsohn’s Elijah with the Mormon Tabernacle Choir. “It’s the very first time in my career that I’ve chosen to do something for myself,” explains Terfel, who was impressed with the quality of the choir when he visited three years earlier. Out of 16 million practising Mormons, 400 are chosen for the choir, and he found the experience deeply moving.

That passion is evident not only in Terfel but also in so many Welsh singers, from the popular Shirley Bassey, Tom Jones, Charlotte Church and Aled Jones to the classically trained Terfel. The Eisteddfod singing competitions throughout the country continue to keep the musical culture alive, and Terfel’s own father, a farmer, and mother, a special needs classroom assistant, both sang regularly in the local choir.

“For the community to come together, they had either religion on a Sunday or they had things like brass bands, or male voice choirs or mixed choirs,” Terfel recalls. “Out of a very hard working-class environment came a certain strength and a platform to nurture talent.”

Music was in Terfel’s family, from his grandparents to his great-great-grandparents, “through choirs and love of singing. My father has most probably got a better voice than I have. But he was never given the opportunity to go somewhere to learn how to sing professionally.”

After winning numerous competitions, Terfel moved to London in 1984, aged 19, to enter the Guildhall School of Music and Drama, graduating in 1989 with the Gold Medal. His career has gone on to include most of the major bass-baritone roles, from his debut with the English National Opera as Figaro in The Marriage of Figaroin 1991, to his forthcoming return as Wotan in Wagner’s Ring Cycle at Covent Garden in October (a production mostly panned by the critics with the exception of his performance).

Terfel confounds the critics by hanging out with pop groups such as Girls Aloud and Jamelia at his own music festival, dubbed Brynfest, organised by him for the past eight years. “When you do a festival like this, you have to give a choice of music, and there has to be one evening which is a certain sell to be able to pay for it,” he explains, agreeing that he also hopes to introduce those who may never go to an opera to classical music.

Brynfest, with its insistence on getting opera singers, commercial artists and 12,000 punters down to North Wales, probably best embodies the tension within Terfel. On the one hand he doesn’t like to leave Wales, while on the other he feels a big responsibility to be Wales’s ambassador for music. His love of home and family is evidently heartfelt.

Terfel’s packed schedule is planned five years in advance. He claims not to feel the pressure of having to perform or even the pressure of fame, although he is occasionally assailed by voice problems and back pain which he puts down to sports injuries sustained when younger. Anyway, Terfel says sanguinely, the back pain has meant that he had more time to spend at home.

He says that being a father is more important to him than any opera he has ever appeared in, and he met his wife Lesley at primary school. (I suggest this is “cute”. He reflects: “Cute? Or destiny, perhaps.”) His eldest child attends the same comprehensive school that he attended “with most of the same teachers still there” and many of his classmates who teased him for going off to be a singer are now joining local Welsh male voice choirs. His wife isn’t keen on “the business” and Terfel relishes their summer holidays in Spain.

Throughout our interview, his only fit of pique is when talking about missing the births of his first two children. “The opera company had forced me to be there for that rehearsal in the morning,” he says of the first birth. For the second, he remembers being in rehearsal with Jonathan Miller: “The opera house should have said ‘Bryn, thank you for even thinking of being here, but go home for six weeks.’ But of course they wouldn’t do that, they wanted their pound of flesh.”

Those words may read more bitterly on the page than they sounded in person, where his deep, honeyed voice made the injustice melt away in the telling. On the way to Bangor station in his soft-top Mercedes convertible, I ask him what singing does for him. He says modestly that it is just another job. I remind him about all the letters he receives from people who have lost loved ones and play his music at their funerals.

Then Terfel concedes that he could “see the depth of feeling for music and the people who perform it”. Your work moves people deeply, I tell him. “It moves me too,” Terfel says.

Sweeney Todd, The Demon Barber of Fleet Street, Royal Festival Hall, July 5-7 (0871 663 2500; www.southbankcentre.co.uk)

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