The new star of BBC1's Casualty spin-off, Holby City, is so far removed from the monosyllabic stiffs who populate our screens and the pages of OK! magazine that he had never even heard of the show he was auditioning for.

"I don't really watch television," he says, sounding slightly sheepish. "I'd heard it was a spin-off of Casualty, which I had heard of, and I was like, well I don't know. I was just worried I wouldn't like the people or the work, and then I'd be stuck. But I'm enjoying it a lot more than I expected."

It doesn't take a genius to see why Sheffield has been cast as Alex Adams, the show's sexy new "cardiothoracic senior registrar" (hospital-drama speak for a heart doctor). Although a "confirmed bachelor" in the truly modern sense, he has the sort of looks that have girls endlessly trying to convert him.

"I haven't done any sex scenes yet," he says. "We're into episode ten and there's not a mention of it. I think Michael French's surgeon, who I have replaced, was a bit randy - his character had something in his past that gave him commitment issues. This is a deliberate change of direction."

Although quietly confident about his ability to act the part, it is the inevitable media exposure that Sheffield is rather dreading. "I'm a bit anxious about it all, to be honest," he says, lighting a cigarette. "I quite like my life. I have a nice life and would really rather it didn't change an enormous amount.

"I don't really know of anyone who has been 'out' at the beginning of their career and who has done something as mainstream as this," he says. "No one knows what happens in these sorts of scenarios or situations. I just hope people are open-minded enough not to care."

Already familiar to MTV regulars as Natalie Imbruglia's love-interest in her pop video Torn, (not to mention the dashingly dishy, water-drenched date in the Martini ad where the "weekend started two hours ago") Sheffield is used to a certain amount of man-in-the-street recognition.

"It mainly happens abroad," he says. "But this time people will know my name. I just want to be as honest and open as I can be without talking about my private life to any great extent."

The son of a drama school teacher, Sheffield, now 34, was born and brought up in Coggeshall, Lovejoy country. He had a typical Essex boy upbringing until the age of about five, when he followed a girl called Victoria to ballet school.

"I wanted to go wherever Victoria was going," he says. "I enjoyed it for a while, but then, as children do, I lost interest and wanted to watch telly instead. But after changing my mind one too many times, my mother told me she was fed up with cancelling and I had to decide what I wanted to do. So I went."

Sheffield clearly showed talent, because at the age of nine he won a place at White Lodge, the Royal Ballet training school in Richmond Park. "It's a weird decision to make because you're nine years old and you're deciding about your entire future," he says. Having survived the gruelling training programme, he went on to dance with the Royal Ballet for the next nine years, touring some of the most glamorous opera houses in the world.

"When I got to the age of 20, I started to wonder: if I could choose my life all over again, would I have chosen this?" says Sheffield. "I started to doubt it more and more. I lost the love of it, really. And without the love of dancing there is no point in doing it, because you're not going to earn any money. You'll just end up crippling your body. You have no future. It doesn't train you for anything else and you retire at 35 because you can't keep going. It takes up your whole life, you have no free time.

"The only reason to do it is if you love every minute of it. And I just didn't. I understudied a lot of leads and did the middle ranking stuff - I was a first artist. But it's a very institutionalised and insular world and unless you are one of the top principals on the planet, you have very little, almost no, artistic input into what you do. You are doing choreography that has been done 100 times before, which you can not change. There is only a tiny amount of room for interpretation and, to be honest, it's not asked of you. I felt like I was being treated like a child."

Eventually, in 1993, injury forced Sheffield to leave and he started to make commercials (some 67 in five years). He then graduated to film, landing parts in Merlin and Anna Karenina, as well as television, doing ITV's The Governor. His new acting career took him to the RSC in 1997 as Petroclus in Ian Judge's Troilus and Cressida.

His years at the Royal Ballet may be a thing of the past, yet Sheffield still has something of the dancer about him. Not only does he look like he could grand jeté with the best of them, ballet also influences his attitude to work. "Most people live their lives the other way round," he says, smiling. "I knew what I was going to do at the age of nine. I was very goal orientated from an early age. But if you asked me what I wanted to do after this, I wouldn't want to say. It's not healthy to be so decided about what you want to do. You are not open to new experiences.

"I'm trying to be a little bit more go-with-the-flow, because my natural instinct is to be goal orientated." He leans back and smiles. "It's my very middle-class Essex upbringing."