Get a photographer quick. Amanda Roocroft is smiling! In nearly 20 years of watching her on stage, I don’t think I’ve ever seen it happen before. “Well, I’ve never done a happy role before!” she says. “What’s more, in this show I get to wear gorgeous gowns, instead of daggy cardigans and bed-socks.”

It’s true. The posters plastered over the Tube for the new Merry Widow prove it. There is Roocroft – the face that launched a thousand anguished Janácek and Tchaikovsky operas – wearing a cat-got-the-cream smile and the most sumptuous fin-de-siècle frock. All dolled up, in other words, to play Hanna Glawari, the fun-loving widow herself, when Franz Lehar’s divinely decadent 1905 operetta opens at English National Opera tomorrow.

“I love that girl,” Roocroft says. “To be Hanna Glawari in real life would be wonderful. Especially with the £20 million stashed away. Mind you, the role is completely out of my comfort zone. Delivering spoken dialogue is very scary. No bar lines! Nothing to tell you whether the voice should go high or low.”

In this show Roocroft finds herself, improbably, playing on the same stage as the veteran comic Roy Hudd. But does she have any funny lines? “No. Well, I don’t think so. If I do I’m not speaking them very well, because nobody laughs.” She bursts into a peal of giggles.

She grew up in Chorley, and played the cornet in the local brass band. “I loved the camaraderie of brass bands. One great highlight of my life was singing a concert a couple of years ago with the Grimethorpe Colliery Band. I’m sure my love of male company comes from that upbringing.”

She has never lost the Lancashire accent. But will it be tamed to play the wealthy Viennese widow? “No. It’s northern vowels all the way. After all, Hanna is a farmer’s daughter. She’s got a real mouth on her. That’s a quality I like. But at the same time there’s a vulnerability about her that appeals to me. She’s been desperately hurt in love.”

Roocroft can relate to that. From the time that, as a young soprano at Glyndebourne, she was the cause of a much-publicised punch-up between an overamorous star tenor and her boyfriend (who was singing in the chorus), her own love life has rarely been simple or settled. Her two failed marriages have produced three children and a ton of heartache.

“I say to my boys, who are 12, 8 and 5, that I’ve been very good at choosing my friends. My husbands [one a singer, the other a vocal coach] were friends. But suddenly when we married they ceased to be friends. And as soon as we ceased being married, we became friends again! I’ve been told – by them – that it’s very difficult being my husband. So there you go. It was best that I put them out of their misery.”

She maintains that, when tackling such anguished roles as Jenufa, Butterfly and Tatiana in Eugene Onegin, she did draw on her own inner turmoil. “There have been times, yes, when I’ve used a role to convey my own sadness. Sometimes that’s been very useful. Being British, I wouldn’t necessarily express what I’m really feeling in real life. So I feel privileged that I can go into a rehearsal, put on a tragic role and pour it all out.”

Isn’t that perilous? “Yes. You have to recognise if you are going too far.

It’s a knife edge.

And of course the stress I’ve been through – ten years of turmoil, really – s o m e t i m e s had an effect on performances. Without exception every opening night during those ten years was a turbulent occasion. I don’t miss that. In fact I don’t know how I made it this far without having a nervous breakdown – except that opera has been my saviour. I know that I’m now enjoying singing more than I have for a very long time.”

She attributes some of that new stability to becoming a born-again Christian. But is it also significant that, for the first time in years, she is unattached? “Yes. I’ve told my friends that they mustn’t allow me to get into a relationship! For the first time I’m not having to fit into another person’s life.”

Roocroft has never been the most consistent singer. I remember nights – when she sang in Oneginat Cardiff a few years back, or more recently in ENO’s Jenufa – when everything clicked and you couldn’t imagine a more convincing, complete and overwhelming interpretation of the role, vocally or dramatically. But on other occasions the complex physical and psychological mechanisms by which opera singers operate didn’t seem to cohere at the right moment.

“I was frustrated for years,” she admits. “It sounds poncy to say this but when I was reading the score of an opera I could hear in my mind how my character should sound but not do it for real. Then I worked out why. It was fear. I didn’t let myself sing fully. Then, if I felt I’d failed, I would beat myself up. And to be beaten up only compounded my misery. But I’ve realised now that there’s no such thing as perfection. That’s God’s job. And that realisation has taken all the pressure off.”

She was acclaimed at a dangerously young age – famously tipped for stardom in 1989 while still at music college, then briefly given glossy promotion by EMI while still in her twenties. Was that also part of the problem?

“Completely,” she says. “It’s tremendous pressure. I wouldn’t wish it on my worst enemy. There were times when I just wanted to fall off the face of the Earth. But the fact that I didn’t sink is something I’m proud of. I was written off so many times. People said: ‘She won’t last three years.’ Then, after three years: ‘She won’t last another three years.’ Well, I’m 42 now, and still here.”

And on the crest of a wave. After The Merry Widow she will tackle Ellen Orford in Peter Grimes (goodness knows what inner turmoil she will find in that long-suffering lady’s psyche), Desdemona in Otello at WNO and Capriccio at ENO. She’s also in a short Janácek opera, Osud, at the Proms.

That, I comment, is a wonderful little rarity. “Yes,” she replies. “Unfortunately I die horribly in Act II.” Back to business as usual, then.

The Merry Widow opens Saturday 26 April 2008 at the London Coliseum (0870 1450200)

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