The essential Isolde

There were many fine things about that performance of Act One. Donald Runnicles conducted with masterful sweep and nuance. The orchestra made a sound that was truly sumptuous. Tenor Ian Storey sang stoically, though Act Two is the real test for Tristans (and American tenor Robert Dean Smith has been drafted in to replace the indisposed Storey for this week's concerts). Even the cameo role of the Young Sailor was beautifully judged by Nicky Spence. But it was Isolde who really turned heads back in September. "Resplendent," I wrote in my review, somewhat breathlessly. "Saucy, tempestuous, tender, the Swedish soprano is a potent force on the concert platform. With one sideways glance she can sum up a whole raft of choreography."

I could have written so much more. Nina Stemme is one of the world's great Isoldes. Her dark-hued voice is magisterial in proportions, her dramatic capacity is devastatingly emotive and her stage presence has the ability, without histrionics or cheap tricks, to stop everyone around her in their tracks.

And yet Stemme was a latecomer to the opera world. She first trained as an economist, and even when she pursued singing seriously her voice took time to solidify. Partly this was because she initially took mezzo roles – "I have a low speaking voice," she says (I'd call it more hearty than low), "and a healthy middle range when singing, so some people assumed I must be a mezzo. But I got very tired in those lower roles. It was only when I became a soprano that everything fell into place. I had to rework my entire voice, note by note, and didn't sing in front of anybody for about a year and a half. That was a test."

Her big break came in 1993, when she won both Placido Domingo's singing competition Operalia and BBC Cardiff Singer of the World. "I was 30 then but my technique still wasn't firm," she says. "I still had to be very careful what roles I chose."

Drinking coffee in her dressing room, effortlessly elegant in jeans and a jumper, Stemme comes across as a very grounded and pragmatic individual. "At the end of the day I'm the only one who's responsible for my voice," she says. "People will just try to use it. They tried to get me to sing dramatic repertoire too early, but I said no. And one no gives you more respect than one yes too many." Wise words indeed.

After her competition success she was offered a contract at the Vienna State Opera, but turned it down. Instead she went to Cologne, "where I could learn all the major Puccini and Verdi roles properly and in my own time." Her voice grew and grew. "I tried to squeeze it in with a shoehorn to roles like Pamina, but for every bigger role I took on, my voice grew with it."

Her colleagues warned her that once she reached Isolde and Brunnhilde there was no way back. "They were right. Of course I can sing other roles, but people don't ask me to. If I was to play the Countess [from The Marriage of Figaro] they'd need to cast the whole production with voices of my type."

So Stemme is stuck with the heaviest roles in the operatic repertoire thanks to the sheer dimensions of her voice. "My heart is still with Puccini and Verdi but my voice is with Wagner and Strauss," she says, a little wistfully. "Verdi was the music that opened my heart and soul to opera. Whereas Wagner has to come to you; you can't force yourself on to Wagner." Incidentally, she sees teenagers as the prime Wagner audience: "His music speaks to their emotions and their searching state of mind –this kind of instant love that's so overwhelming you don't need any potion but that you don't admit out of pride or loyalty or whatever."

She finds Wagner "endlessly fascinating because there are so many emotional layers to explore. Verdi had the same depth, of course, but he's Italian so it goes much quicker-" And she takes pleasure in bringing touches of Puccini and Verdi to the way she handles German music. "Wagner wanted his vocal lines to be sung bel canto. Bringing Verdian colour and phrasing, tiny bits of portamento – these are things I work with endlessly in Wagner."

Stemme has sung the role of Isolde more than 60 times but says she never tires of it. "Never. This role is like the end station. It's eternal because you can work and work on it. What is there after Tristan und Isolde? It's the ultimate challenge. I discover something new every time I perform it."

And performing Act Two in isolation should be no problem, she says – except that she might have to warm up a little more than usual. "The first act is like an opera in itself, and is normally a great warm-up. The second act is higher, more jubilant, crazier."

The intervening six weeks in this production will just feel "like a very long interval," she laughs. Anyway, Wagner's score doesn't specify the length of time that passes plot-wise between the acts. By the beginning of Act Two Isolde has married King Marke and lived with him a while. Some versions of the legend tell of Isolde trying to escape, of her and Tristan finding multiple ways to fool Marke so they can meet. In effect, then, the BBC SSO's "long interval" is simply a way of letting the story play out in real time.

"And besides," says Stemme, "what is time in Wagner? His works subvert any normal sense of the word. His endless stretches don't have anything to do with minutes or hours or even months. Waiting six weeks between acts will make little difference to the musical experience."

Nina Stemme sings Act Two of Tristan und Isolde with the BBC Scottish Symphony Orchestra at City Halls, Glasgow, tomorrow and the Usher Hall, Edinburgh, on Sunday. Act Three is in Glasgow on April 11 and Edinburgh on April 14.