It's amazing what three-quarters of a bottle of wine can do for a singer. Take the soprano Rosa Mannion, who tonight sings all four of the female leads in the ENO staging of Offenbach's Tales of Hoffmann. She had wanted to be the next Florence Nightingale, but then "I went completely berserk and said `I want to audition for music college now.' " So it was that she was called up on short notice - she was having a barbecue at the time - to cover the famously difficult Constanze in Mozart's Seraglio for Glyndebourne.

"The barbecue debut, I think it was called. Put the sausages back in the fridge - I'm off! I'd never been on stage at Glyndebourne before, I was 26 or something, completely green about the gills, and waltzed on there half-cut. I think it's the best Constanze I've ever done."

Then there was an unusual Mozart production. "One of my best friends in the whole world, Amanda Roocroft, and I were Fiordiligi and Dorabella in John Eliot Gardiner's Cosi. He wasn't just conducting it; he was directing it. And he had a major concept: the two sisters would be interchangeable. One could happily come out with what the other was thinking. And John Eliot wanted that in terms of singing. His original idea was, `I'll say to you in the wings, you're singing Smanie implacibili tonight - to Fiordiligi - and you'll be singing Come scoglio - to Dorabella' ".

"We sort of took a deep breath and said, "Interesting ..." - in absolute fear and terror as to whether we warm up to top Cs or not on the night. Then he changed his mind and wanted us to try phrase about. One would start an aria, sing a phrase - then the other would take over. We finally compromised: we would each have one phrase of each other's aria to sing. We enjoyed it, and worked out a few gags round it - and we've actually got that on the CD. Some people can't tell which phrase it is, which I think is fascinating."

Mannion's biography lists the "immense personal success" of her first Violetta for Jonathan Miller's 1996 ENO Traviata, a production that otherwise had a lukewarm reception. "Well ...", begins the soprano, "I am still astonished by the reaction. What I adored about it was its simplicity. It was so different from any of the other Traviatas around. And it focused so much on the characters. When Jonathan first said to me, `You're going to do the whole thing in bed', immediately I felt, `Absolutely! That's exactly right'.

"Jonathan based his ideas on photos of the salons of the time. We wanted very much to create that atmosphere: she wasn't just a floozy, she was a very intelligent courtesan - she needed to be because of the clientele she had. We did try different versions of the costume - overtly sexy, floaty blouses, open, with basques underneath ... in the end we just went for the simple vaguely boyish look. That is the role that has meant more to me than any of the others - it completely and utterly consumed me."

Dramatically, what does she bring to rehearsal? "On the first day, as little as possible in terms of expectation and ideas for character. Obviously the music is your biggest clue. The music gives you 80 per cent straight away. Then it's between you and the director. If you have a good dialogue, then you're laughing. If you have a director who has a fixed idea of what you've got to do, and won't listen, that's a nightmare."

Now comes Michael Kaye's much-researched new edition of Hoffmann. It has spoken dialogue, with the love-interest appearing in the composer's preferred order of Olympia, Antonia and then Giulietta, with Stella's presence hovering over both prologue and epilogue. "They're all components of this one personality - Stella - which causes him all this angst. It makes great dramatic sense to do it this way. First, Hoffmann falls in love with this empty shell, the doll. The second one is the virgin, pure, who drops down dead before their love can be finalised. And then he descends into the depths, tries to throw his lot in with Giulietta the whore before abandoning women altogether.

"It has presented me with a massive challenge. There is no warming up to play The Doll (Olympia). You come on and you have to sing this phenomenal firework coloratura thing. It's a purely technical and vocal thing, which I always find hard. Antonia is a joy to sing, but it's so short - like a complete little operetta all in one. You don't have any time to develop - you just have to get right in there. Giulietta, in editions that have been done before, has been a mezzo. This one is a coloratura soprano. She has a firecracker aria, which we have reinstated.

"None of us is used to doing dialogue and we're having to project into this massive great auditorium over orchestral accompaniment. So you use the same kind of projection and support as you do when you're singing - not on your throat, but on your diaphragm. That takes a lot of getting used to - as does using different vocal colours. Giulietta is a hard-bitten materialistic whore. I use a lower speaking voice for that, to make her more sophisticated and older. Antonia has to be quite young and breathless."

The future for Mannion sees her in Handel's Rinaldo opposite Cecilia Bartoli, a return to Glyndebourne for her first Elvira and, very soon, Manon at ENO. "Singing all these roles, it's therapy - because you get rid of all your frustrations and everything on-stage. It's all legal - it's great!"

`Tales of Hoffmann' opens at the ENO, The Coliseum, tonight at 7pm.