A few years ago, Depp got a call from Burton. “He said: ‘How do you feel about ‘Sweeney’? You’d have to sing. Do you think you can do it?’ ” Depp didn’t know, but he started listening to the dissonant harmonies, syncopated rhythms and painful tunes without lyrics. “To see if I could hold the note,” he said. “Could I bend the note? Did I have anything to add? It was a bit of experimentation.

But I called Tim and said that something could be done.” Indeed, something was done — something conceptually large, haunting, ghoulish, ironic and dripping with Grand Guignol melodrama. It happened at London’s Pinewood studios last winter: The friends assembled with Burton’s companion, Helena Bonham Carter, Sacha Baron Cohen, Alan Rickman and others to turn Sondheim’s masterpiece into a desaturated nouveau-horror music-film that will open in theaters Friday.

“Sweeney Todd” isn’t just a work about splintered morality that requires the ability to sing. It’s high culture, full of deliciously bitter contradictions, made of concert-caliber music, sharp words and a pentameter that would send mainstream theater singers running for a “Wicked” audition. And it’s low culture, based on a 19th century legend of a serial killer who slices throats.

But ask some Sondheim purists about a Hollywood version — this one, with a beefed-up 78-person orchestra and the opening ballad cut — and you might get scoffs. The key, therefore, was not to produce a performance film like Ingmar Bergman’s “The Magic Flute” but to create an original movie genre: a consciously present-day spoke-sung music-film with younger actors, no traditional singers and a cinema-grotesquerie style, full of viscous slashes of blood.