Jason Donovan is taking a risk by playing Sweeney Todd.

Steven Sondheim's bloodthirsty musical thriller is far removed from singalongs such as Chitty Chitty Bang Bang or The Rocky Horror Show - Jason's most recent stage exploits - and the demon barber of Fleet Street is his most challenging role yet.

Set in London, Sweeney Todd tells the dark and twisted tale of the murderous barber who kills his customers and forms a sinister relationship with downstairs pie shop owner Mrs Lovett, played by Harriet Thorpe - better known as the dippy receptionist Carole in TV comedy The Brittas Empire.

"I am looking forward to it, as there is a strong emphasis on the acting," says Jason. "It's just very different to what I have done before.

"My father was an actor, and I am first and foremost an actor who sings. It's where I'm most comfortable.

"This is a good moment for me because I wanted to move on," he continues. "It's an opportunity. Not that I disrespect anything I've done in the past. I'm proud of those moments, but it's just that I am a bit older and I've been through darker moments as well. I rise to the challenge."

During the late Eighties and early Nineties, Jason's boy-next-door image was a carefully-controlled commodity and a big part of the early success in this country of Neighbours, the Australian soap opera that first brought him into the public eye. But this cleancut image proved to be both a blessing and a curse.

Trading on his boyish good looks was great for Neighbours' ratings and the record sales of the pop career he quickly launched on the back of his success as an actor. At the peak of his fame in the late Eighties he sold 13 million records and had four number one hits. His first single, Especially For You, a duet with Kylie Minogue, was released in 1988 and following the release of his second album in 1989, he became the biggest-selling artist in Euprope and the UK.

But things took a drastic downturn when he decided to sue style bible, The Face, in 1992.

The cooler than thou magazine famously outed him, plastering posters all over London with his picture and the words Q***** as F*** splashed across it.

Despite waiving the damages he won in court, Jason was branded uncool and homophobic, was shunned in celebrity circles and eventually dropped by his record label. Meanwhile, as his own career nose-dived, his former girlfriend and Neighbours co-star, Kylie, rose to super-stardom.

For a time Jason admits he felt bitterly disappointed and began taking drugs to "mask it", collapsing several times in London nightclubs.

"I was never deluded that things were going to last forever," he says. "But to say I wasn't disappointed would not be true.

"But I went into a different phase," he continues. "I started to explore a different part of myself - the good and bad I was doing to myself. I wanted to f*** the canvas up a bit.

"I wanted to be Kurt Cobain there for a while," he laughs, "but then you realise again you're trying to be like someone else."

Now a father of two, clean of drugs and in a stable relationship, he says: "I'm a far stronger person, far wiser.

"As long as I'm working I'm a happy person - as long as I'm involved producing something. I'd still love to write a good record. I'd love to make a good film. But I'd also love to make a good musical.

"I'm not desperate to become the biggest star in the world. I don't want to say I don't have ambition because I do, but do I need 20 rooms in my house? Not really.

"I've been lucky to have success at a younger age. There are a lot of people who are very talented who haven't had the opportunities I've had. Do I walk around bitter and twisted about the fact I haven't sold a million records in the last ten years? No, not really.

"I've been lucky and I'm financially stable enough not to worry about it."

Ray Winstone, who starred as Sweeney Todd in the BBC's recent television adaptation, said he found the key to the character by tapping into his weakness. Sweeney Todd is a tortured soul who was abused in childhood - he's not a monster. His actions stem from deep-seated psychological problems.

Harriet agrees. "You only have to read the news to see Sweeney Todd is not all that far from any of us," she says. "Sometimes people do these things out of desperate love and passion.

"It's a ghoulish, ghastly tale and it's also about love. Sweeney Todd's loss of love, the inappropriate love of the judge for Sweeney Todd's young daughter, her love for a young sailor, Mrs Lovett's love for Sweeney Todd. It's a terrible, thwarted love story."

"I'm sure not all murderers walk around like this," adds Jason, assuming a mock-murderous stare. "That's the cliched version. At the moment I am still a bit of that," he jokes, "but I will find the depth of the character.

"The Ray Winstone one is very different. I found it a little depressing - it's not a criticism, that's probably how the story is - but ours is more romantic. The music is beautiful, there are some gorgeous love songs. I think it's more interesting."

Stripped down to a sparse set and minimal cast, John Doyle's acclaimed Broadway production is presented as though perfomed by a group in an insane asylum. Instead of an orchestra, the actors themselves play the music with various instruments on stage. Jason likens it to a Herb Ritz photo. "It's very black and white," he says.

"It's a very exciting, dynamic production," adds Harriet. "People will definitely leave on a high. The contemporary costumes mean the audience instantly, instinctively engage with the characters.

"The props morph into different things - a coffin becomes a table, then a judge's bench and then a balcony. The audience has to participate in this and so it's very intoxicating.

"It's make-believe and this touches people's creativity. The audience becomes part of it."

Starts 7.45pm. Tickets cost £16-£27. Call 08700 606 650