Sprawled forlornly on a sofa, Tim Burton splutters his British cold into a British handkerchief. He has lived in Hampstead for years, and has a London-based family, so you’d think this LA transplant could lay claim to having “gone native”. (On cue, his girlfriend, Helena Bonham Carter, nine months pregnant and with apparent shared enthusiasm for Burton’s dragged-through-several-hedges-backwards coiffure, waddles through to the loo.) There are still gaps to be plugged in Burton’s local knowledge, though. Did he understand that Sweeney Todd was cockney rhyming slang for Flying Squad? “No, I did not,” he says, pepping up. He’s more than familiar with the old cop series. “Yeah, yeah, yeah,” he enthuses. “So that’s what it relates to?”

The maverick director attends, instead, to the tale of the demon barber of Fleet Street – the dreaded Victorian barnet-buzzer who, according to lore (and The Two Ronnies), would slit the throats of his customers without so much as an “Anything for the weekend, sir?”, their carcasses dispatched to the pie shop downstairs to be minced up as filling by the kindly Mrs Lovett. “You think,” he asks, “he was a real person?”

Burton, as it happens, doesn’t believe Todd existed – “It’s sort of an urban myth,” he concedes, disappointed. Quite possibly there was some psychotic shaver who inspired the fable, his misdeeds, given currency by the penny dreadfuls of the era, bringing him within an untrimmed whisker of the notoriety of Jack the Ripper.

But veracity doesn’t matter. Given that Burton’s quirky flicks have celebrated both oddballs (Pee-wee Herman, Edward Scissorhands, Ed Wood, Willy Wonka) and the Hammeresque macabre (Sleepy Hollow, The Nightmare Before Christmas, Corpse Bride), you can see why this story would have him jumping all over it.

This month gives us his $50m gothic opera (almost literally), starring Johnny Depp as the infamous throat-slasher and Bonham Carter as his pastry-rolling accomplice, with support from the likes of Alan Rickman, Timothy Spall and Sacha Baron Cohen. Shot, as ever, at Pinewood (“Here, I’m more able to focus on the movie,” Burton says. “There [in Hollywood], you just feel this vibe of the business around you”), it’s a handsome beast, Depp’s restyled Todd plodding about like a smack-addled version of the skunk-haired 1970s sportscaster Dickie Davies.

It is also Burton’s most ambitious film to date, for the words “Sweeney” and “Todd” have, in recent times, become synonymous with the 1979 musical by Stephen Sondheim. It is this that Burton is realising for the big screen. “I’d never really done something like this,” he says. “I’d always had music in movies, but never full-blown. It’s very operatic, and almost everybody in the cast is not a professional singer. Even seasoned Broadway people are saying how difficult it is.” You could point out the commercial nono of there being more blood flowing than out of the lift doors in The Shining; that cannibalism has never proven a box-office turn-on; and that, in the States, it’s been slapped with a kiss-of-death R rating. Dare one mention that this particularly dissonant Sondheim score is not exactly High Society when it comes to populist sing-along show stoppers?

At 77, Sondheim remains the most influential figure in modern Broadway musical theatre, his bank account still jingling any time somebody gets up to warble Send in the Clowns. A game-playing polymath (the playwright Anthony Shaffer modelled Sleuth’s Andrew Wyke on him), he is unique in having won an Academy Award, multiple Tonys and the Pulitzer. His hit shows – Company, Follies, Sunday in the Park with George, Into the Woods and Passion – are embraced by the faithful as sacred texts. Only two have crossed over to the silver screen: A Funny Thing Happened on the Way to the Forum and A Little Night Music. Both crashed like vaudeville cymbals.

“The one form of movies that I never particularly enjoyed was the movie-musical,” Sondheim cautions. “I liked the sort of fluffy musicals before the second world war, the Astaire/Rogers things, but movie-musicals that told stories have always struck me as ponderous.” It’s all down to the gulf between “stage time” and “film time”, he explains, the movie medium being unable to accommodate someone simply standing and singing for several minutes. “Take Tonight from West Side Story. It’s a close-up of him, then a close-up of her, then a two-shot, then a shot of the fire escape. There’s nothing to do. You have to waste the time.” We will forgive Sondheim here. He did, after all, pen the lyrics.

For all his stage triumphs, it is 1979’s Sweeney Todd that is regarded as Sondheim’s meisterwerk. The original production garnered eight Tonys and wows for its leads, Len Cariou and Angela Lansbury, with the inventive staging commended for the manner in which Todd’s victims hurtled down a chute from his spring-loaded chair. A Grand Guignol, based on the 1973 play by Christopher Bond, it envisages Todd as the alter ego of Benjamin Barker, a man returning from penal servitude in Australia who changes identity to seek revenge on Judge Turpin, the malicious magistrate who absconded with his wife and has become the guardian of Barker’s teenage daughter. Todd’s vengeance is soon extended to any gent with a five o’clock shadow.

Burton was a humble art student when he caught the London production on his first trip over in 1980. “I wasn’t into theatre,” he recalls. “I’d never heard of Sondheim. I just sort of stumbled on it and it really affected me. The first time on stage I saw them singing Johanna, and with the throat, you know, the blood, I thought, ‘This is a unique juxtaposition of music and image.’” It seemed, he adds, “like a great movie score. It would lend itself to one of those old horror movies”. He was not off target. Sondheim conceived the music as a tribute to the film composer Bernard Herr-mann, best known for his Hitchcock collaborations.

Twelve years later, after Burton had sealed his spot on the Tinseltown A list, he approached Sondheim about a film version. “Then I never heard from him again,” Sondheim mutters. Alan Parker subsequently showed interest, as did Sam Mendes, who went off to shoot Jarhead instead. Burton was already into a year-long preproduction on the Jim Carrey film Ripley’s Believe It or Not! – but when that fell through at the 11th hour, Sweeney Todd landed in his lap again. “In some ways, I think the timing was more right,” he muses. “Because, having someone like Johnny, it’s like 10 years or more of life experience, which kind of informs this version.”

Sondheim’s consent came on condition that he retained personal control over both the casting and the score. Even so, given Burton’s idiosyncratic sensibilities, Sondheim seems to have been generous. Not least because Depp (in his sixth teaming with Burton) had no proven record as a singer, a sticking point for the studio until the actor’s homemade demo of himself crooning My Friends (Todd’s ode to his darling razors) convinced all that he could do it. “The fact that he came from a musical background, a rock band, even though he was not a lead singer, I knew he was musical,” Sondheim insists. (Depp’s voice is a deep, Bowie-Berlin mocker-ney.) “I also knew that he was intelligent enough not to allow himself to play this part unless he could handle it vocally.”

The greatest gift to Burton is that Sondheim has been fully complicit in shoehorning his own three-hour stage version into less than two hours of screen time. “I do not believe that anything is written in marble. I want the story to move ahead,” he says. “The thing with Tim is that he understands that. Where the songs did not either suggest or need a camera, ‘Let’s cut’, Tim would say to me, or [the writer] John Logan, and I’d look at it and see if I could elide it or rewrite so it had film motion to it.”

He lists the various omissions: the absence of the overture, the verses excised from the songs Green Finch and Linnet Bird, and Pirelli’s Miracle Elixir, and the shaving down of the pie eulogy God, That’s Good! “On the stage, it’s about seven and a half minutes long,” Sondheim says, “and it’s a very complicated piece. On the screen, it’s three very short choruses, and that’s all that’s necessary. You get it already. She’s got a successful meat-pie shop. Let’s go on to the next part of the story.” With the score recorded at George Martin’s Air Studios (and lip-synched back during filming), Burton was graciously left to his own devices.

The film opened in America before Christmas, and the critics have given their verdict. Indeed, Burton has been notching up the commendations in the preOscars sweepstakes, bagging, among others, four Golden Globe nominations (best musical or comedy, plus nods for Burton, Depp and Bonham Carter). For the film’s veteran producer, Dick Zanuck, former head of Twentieth Century Fox, drummed out of office for making the 1960s bombs Hello, Dolly! and Doctor Dolittle, it is welcome confirmation that newsof the movie-musical’s demise had been greatly exaggerated.

Sweeney Todd is by no means a cosy bedfellow for the likes of Moulin Rouge! and Chicago, or the more recent Dreamgirls and Hairspray, which seem to stem from a revival launched in 1996 with Evita. The good news is that Sweeney Todd’s creator gives Burton’s film his ringing endorsement – “I was pretty stunned, I must say.” It’s the most important accolade for Burton, whose “big thing”, he says, was “pleasing Sondheim”.

Sondheim is relenting in his old age. There’s a move afoot to transfer Follies to the screen, he admits. “It isn’t so much, ‘Oh, that movie worked, let’s do another of his musicals.’ It’s, ‘Is that story worth telling?’ Not only that, but can it be done on the screen, and be in some way enhanced by being on the screen? One of the things I really like about this movie is that it’s not just a film of Sweeney Todd the stage musical – this is an actual movie.”

It will be the arch Sondheimites who will judge the film most harshly. Burton is dismissive. “I always say: this is a movie. If you want to see the Broadway show, go look at the Broadway show. It’s a different thing.” Still, he knows Sweeney Todd is going to be a tough sell. “From a studio marketing point of view, I’m sure it is difficult,” he says. “Things can fall through the cracks. People say, ‘I want to see a musical, but there’s too much blood’, or ‘There’s just the right amount of blood, but what the hell’s all that singing about?’ It could go that way, or it could go the way I hope it goes, which is that it’s not like something you’ve ever seen.”

He’s working on his own tag line. “The Sound of Music,” he laughs, “with blood!” And, he might add, no Dennis Waterman.

Sweeney Todd: The Demon Barber of Fleet Street opens on January 25

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