There are two sorts of diva interview. The first is when the goddess descends to the microphone ready to play a part, preened for a mix of artful skirmishing (that spat with the director, that missed rehearsal) and entertaining whimsy (the forthcoming tango album, why she’s “just an ordinary girl” at heart). The second kind is a tougher, but more honest, sell: the type who really wants to level with you on just how difficult it has been, how hard she works, and how being a diva is the last thing on her mind.

It ought to be obvious which box to put Karita Mattila into. The Finnish soprano arrives for our appointment in her Royal Opera House dressing room so rouged up that our photographer later confesses that she had misjudged her age by more than ten years (Mattila is 46).

In fearsome heels, the six-foot-plus blonde towers over me in a voluminous and oddly macho brown leather jacket. Five minutes into our chat, and she’s still having her blusher topped up by an ROH assistant. Ten minutes in, and she’s still answering my first question, in a breathless stream-of-consciousness and totally unexcerptable sort of way. So far, so type one.

But it’s a miscalculation. Mattila’s deep, commanding, speaking voice, catlike stare and gnomic pronouncements might give her a Dietrich-style hauteur, but underneath it all is a singer addicted to her craft and surprisingly candid about her foibles and failings.

And if she does ramble about her current obsession – the lead role in Jürgen Flimm’s gritty production of Beethoven’s Fidelio, the story of an heroic woman, Leonore, who masquerades as a man to release her husband from prison – it’s because she cares so much about it. “The realness of it – that’s what touches me as a performer,” she says. “And then you look forward to the challenges of the music because they go together with the part.”

If the New York reviews of Mattila’s Leonore are to believed, she throws herself into this particular part with near superhuman abandon – as well as having total credibility en travestie. But the key, says Mattila, was Flimm. “He treated Leonore as an equal – a fascinating woman, who was smart, who had a plan. It was just about this relationship – what this person who loves her spouse would do, and if something like that happened to me, then I think anyone would do the same thing.”

Finding a role to satisfy Mattila, who craves real women in a genre where they tend to be drawn in broader brushstrokes, is difficult. “But the production is the thing that judges how the role is treated,” she comments, “and usually, the better the piece the more room there is for interpretation.”

But this strident feminist’s greatest triumph is surprising – not a role, but her marriage, to the Finnish car salesman whom she first met when he sold her a Mitsubishi Sigma. “We’ve been married for 15 years and we’ve been together since 1988. That’s the success I value the most. And it really gives me most of my energy.”

Interestingly, it was also in the late 1980s that Mattila finally made her real professional breakthrough, years after this shy and slightly overweight daughter of a Finnish farmer had won the first Cardiff Singer of the World competition. “Well, I’m definitely a happier woman now than when I was 25, and a happier singer. I was so worried then, and I had so many issues.”

Now 46 and chewing up the dramatic roles that suit her best, Mattila is at her peak – though she admits that she isn’t entirely comfortable with the prospect of ageing. “The only thing is, of course, my vanity. Thank God I have my exercise routine. But I’m aware when I see old pictures of myself and I see the difference – and that’s tough, especially in our culture.”

Could this really be the same woman who recently stripped off at the Metropolitan Opera, New York, as the sex-crazed princess Salome? The question takes me into dangerous waters – Mattila has already rebuffed my earlier attempt to ask her about the production of Strauss’s steamy shocker (also directed by Flimm) in which she bared all for the infamous Dance of the Seven Veils. But now she relents. “If [the nudity] had looked to me like it was fake and treated as an effect, just for shock, then it wouldn’t have worked, but if I believe in the concept then it’s not a problem. But if you want to do Salome in the style that I did it, you have to be fitter than you’ve ever been in your life.”

It wasn’t without controversy. “I know I was one hell of a good Salome – I was able to sing it, I danced and I worked like crazy. But it took everything out of me. And then comes the Finnish tabloid press and hints that my success is because I undress. That kind of stupidity is unforgivable.”

For all Mattila’s still prickly anger, the anecdote is less interesting for the row than it is as evidence of her ability to conquer her hang-ups by channelling them through her work. “The fuss for me was long before I started the Salome rehearsals. Then I left the speculating to others.”

Our time is up. An all-important physiotherapy session beckons, but I’m allowed one more salvo. I ask for her future plans – only to hit a surprise. “I’m working on a jazz project,” she beams, “Four concerts, with the best Finnish resources.”

“I’ve Got You Under My Skin?” I venture. “No,” she sweetly retorts, “The Man I Love”. My audience with Salome is over, and I’m left wondering – about the man from Mitsubishi and how on earth he manages it.

Fidelio opens on Sunday at the Royal Opera House (020-7304 4000)

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