I hear Susan Bullock before I actually meet her. And what I hear is pretty disturbing. I am skulking through the vaults of the Royal Opera House; her powerful soprano, with its rich vibrato wrapped around a steely core, is blasting out of the Tannoy on the wall. And, if I know my Richard Strauss, Bullock is currently urging her long-lost brother to kill her mother. With an axe. That her character has been keeping in a hole in the dirt. Which is also where she lives.

Small wonder that I have to wait a few more minutes before Bullock frees herself from the grip of Strauss’s shocking Elektra to meet me for a post-rehearsal sandwich. “She’s worried there’ll be blood on her,” explains the press officer, before it turns out that the stage ketchup is being splattered only after lunch, and the singer who meets me is both neatly coiffured and refreshingly down to earth.

Britain’s premier dramatic soprano has, after all, already done two runs of Elektra this year “with a quick Ring in Budapest for light relief”, she laughs, as if singing in Wagner’s epic tetralogy could somehow be a relaxing break.

But the very first time Cheshire-born Bullock sang the title role in Elektra, which she is tackling both at Covent Garden (opening tonight) and in an unseasonal December run of concert performances for Opera North, preparing for the part was far harder. “I lived the life of a hermit. I couldn’t relax by watching TV or anything. I just stared at the wall. Because it is a very weird thing to be thinking about on a daily basis – murdering your mother, murdering your stepfather and the death of your own father. Now I’m getting better at dealing with it, but it still takes me over.”

And then of course there’s the actual singing. To tackle Elektra, perhaps the most demanding role in the entire soprano repertory, is to do battle with an orchestra of some 100-plus players: the oft-told anecdote is of Strauss himself shrieking at the first rehearsals of the opera: “Louder, louder, I can still hear the singers.”

But Bullock is having none of it. “If you look at the score of Elektra, if you look at the score of Salome, these big pieces are riddled with scrupulous dynamics – piano, pianissimo, diminuendo, all the time! Strauss knew what he was doing. He didn’t just want to blow you off the stage, and have your larynx flying to the back of the stalls. And I think I would prefer, as a listener, not to be screamed at for one hour and 45 minutes.”

Perhaps this is the secret of Bullock’s newfound facility with, as she puts it, “the large ladies” of the repertoire – Elektra, Salome, Isolde, Brünnhilde: respect for the score, diligence, careful husbanding of resources. Because in the opera world she is a one-off – a former lyric soprano who sang light, fluffy parts (“Gilda, Pamina, Marguérite”), most often at English National Opera in the 1980s and 1990s – but whose voice then took off. Put simply, it didn’t just become better, it became bigger, too. “It’s wonderful to have the opportunity to develop like this, because some voices stay as they are. In a way I’ve had two goes. It’s all come as rather a marvellous surprise.”

Chalk it up to sheer professionalism rather than just luck: even now Bullock Mk II never overtaxes her voice, and has short shrift for a younger generation of opera stars, currently dropping like flies from illness, stress or overwork. “Preparation is nine tenths of the job, because you can’t turn up somewhere, get off the plane and sing a role that’s not really sung into your voice. It’s got to be absolutely in your muscles before you dare turn up to rehearsal.” Fitness helps, too, so it’s handy that Bullock has worked out a way of keeping trim while also learning her lines – on the treadmill. “It’s about 40 minutes for the whole of Salome,” she jokes, “ Götter-dämmerung’s a bit longer – you have to do the bike and the treadmill for that one.”

Perhaps the only time that Bullock really threw caution to the winds was when she jumped into Die Walküre last autumn at Covent Garden at frighteningly short notice. Their Brünnhilde had cancelled the day before; Bullock was singing the same role in Lyons, but had a free couple of days between performances and suddenly found herself jetting off to London to fill in. “It was one in the morning before I had made all the arrangements and I was on a plane at 8. It was the maddest thing I’ve ever done but as long as I live it will be the most exciting.”

It also marked the first time that she had sung with Plácido Domingo. “And I was standing there with this sword, pointing it at his stomach, and he was looking at me so intently. And just for a millisecond I thought, ‘God, I’ve got all your CDs’.” If all goes to plan, London will hear her complete, planned Brünnhilde next time the Ring returns to the Royal Opera House in 2012.

All of which makes Bullock pretty happy: it took her long enough to arrive at Covent Garden, and now she’s their Brünnhilde of choice. What could possibly irk her now? “I’m very satisfied,” she says contentedly. “I have no axe to grind. Ooh, that’s very topical.” She reaches for a sandwich: soon it will be time to return to that hole in the mud.

Susan Bullock sings in Elektra at the Royal Opera House, WC2, from tonight (www.roh.org.uk 020-7304 4000) and in touring concert performances for Opera North from Dec 11 2008 (www.operanorth.co.uk)

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