By Natalie Jamieson Entertainment reporter, BBC News

Sweeney Todd: The Demon Barber of Fleet Street marks director Tim Burton's sixth collaboration with actor Johnny Depp, but it is the first time either has ever attempted a complete musical.

The film tells the tale of a barber-turned-serial killer who seeks revenge on a judge who wrongfully imprisoned him.

Tim Burton is famed for his often macabre films

Charlie and the Chocolate Factory may have shown that Burton and Depp can take on twisted song and dance routines, but it is fair to say The Sound of Music would not fit comfortably into their vision of the world.

But Sweeney Todd, based on Stephen Sondheim's 1979 musical, is suitably nightmarish for the dark duo.

"Oh we're the Astaire and Rogers of the modern age," jokes Burton, whose previous films with Depp include Sleepy Hollow and Edward Scissorhands.

"We were just talking about doing Cats," quips Depp.

The dark world of Sweeney Todd is the stuff of urban legend. His thirst for revenge compels him to murder people with a particularly close shave and dispose of the bodies in meat pies.

This means that young fans used to seeing Depp as Captain Jack Sparrow from Pirates of the Caribbean will have to wait to see him in his latest role.

Johnny Depp and Helena Bonham Carter star in Sweeney Todd

Due to the amount of blood spilled, the film - which also features Helena Bonham Carter, Alan Rickman, Sacha Baron Cohen and Timothy Spall - has been given an 18 rating in the UK.

Burton thinks the censors may have been oversensitive.

"For me I think it's a harsh rating. It's a fantasy you know. The blood and all is meant to be impressionistic as opposed to realistic."

He made a point of casting actors that could sing, rather than singers who could act and even made his old friend Depp and Bonham Carter - the mother of his two children - audition for him.

Depp has been in bands in the past, and even recorded a couple of songs with Oasis, but that was playing guitar.

He admits singing was a new challenge, but did not take any lessons to improve his voice.

"No, I probably should have but I didn't," he explains.

Timothy Spall describes the film as a 'Victorian gothic horror'

"I just figured the best way to learn how to sing as the character, was to learn the character you know. And then the tone, the warbling, whatever, would follow."

Filming shut down at one point so that Depp and his partner Vanessa Paradis could take care of their daughter, Lily-Rose, who was hospitalised with a serious infection. She made a full recovery.

He credits her with some of his musical knowledge - at least his familiarity with High School Musical. "You know I have an eight-year-old daughter, she's almost nine, so I get to watch all those."

Disney's High School Musical franchise and Hairspray starring John Travolta may be recent examples of successful musicals, but they are not to everyone's taste.

Even Spall, who plays a sleazy henchman in Sweeney Todd, admits he does not enjoy that type of movie very much.

Alan Rickman (r) plays the judge who wrongfully imprisons Todd

Yet in the same breath, he says: "Mary Poppins is one of my favourite films. And Guys and Dolls is one of my favourite films so I must like them".

The teaser trailers running on TV for Sweeney Todd have no singing in them whatsoever.

"This isn't really a musical," Spall continues. "It's like a Victorian gothic horror that happens to be sung rather than spoken."

From the opening scenes of the brooding barber returning to London on a creaking ship covered in swirling mist, it is obvious you are watching a Tim Burton creation.

Even before Bonham Carter's character decides to use human flesh in her pies, you know you should not trust her cooking.

This Old England is not an appetising place, although it did give Depp another chance to roll out his now practiced cockney growl.

"It's always fun doing an accent and hopefully I do it well," he says.

"You know I've spent so much time over here it's kind of become like second nature. Somebody actually asked me when I was going to play an American again."

'Uncomfortable'

So how does Depp feel watching himself on celluloid belting out a love song to his character's much-loved razor blades?

"I don't like watching myself - I just feel uncomfortable," he admits, nervously fiddling with his shirt-sleeve.

In fact, he has not even seen the movie yet.

"I will one day. I look forward to seeing Tim's work," he adds, throwing his director buddy a grateful glance.

"But just thinking that I'm in it, I cringe. I love the experience, I love the process - you know I love the whole creative partnership, that's all wonderful.

"But the actual seeing the finished project - if I'm in it - is always a problem for me."

Sweeney Todd the Demon Barber of Fleet Street is now showing at UK cinemas.