Whether it is greeted with acclaim or derision, British director John Doyle’s new Broadway staging of Stephen Sondheim’s Grand Guignol classic “Sweeney Todd” is destined to be one of the most talked about productions of the fall theater season. What other show, after all, impelled its entire cast to join the Broadway musicians union? Which other Broadway musical boasts four pianists? And, perhaps most compellingly, where else in the world can you see Patti LuPone hoist a tuba?

The premise of Mr. Doyle’s directorial conceit is simple yet mind-boggling. He has pared down the cast of one of Sondheim’s most expansive works to 10 cast members (the original had 27) and has dispensed with the orchestra – or, rather, he has turned his ensemble into the orchestra. Vengeful Victorian barber Sweeney (Michael Cerveris) strums a guitar. His ruthlessly practical accomplice, meat-pie purveyor Mrs. Lovett (Ms. LuPone), makes with the oom-pahs. Lovers Johanna and Anthony both bow a cello. Some actors play three instruments, and occasionally switch mid-song. Moreover, all of them go pit musicians one better – for, in addition to committing their acting and vocal parts to heart, they have memorized the score.

John Doyle calls the technique “actor-musicianship.” An outside observer might call it an actor’s nightmare. But the director has applied this approach many times before in his native England, with shows from “The Gondoliers” to “Fiddler on the Roof” (“Sweeney” was first a success at the UK’s Watermill Theatre, and then the West End). His shoulder is ever ready to support a tearful thespian’s head. “I tell them, ‘I’ve seen people go through this before, and of course it’s overwhelming, and of course you probably want to cry right now,” he said.”I promise you’ll come out the other side and you’ll feel okay.’ And they do.”

For the audience, the grisly, blackly comic story of human corruption and Sondheim’s grandiose score still fascinate, but so does the labyrinthine web of lines, blocking, and musical cues. Alexander Gemignani, who plays the villainous Beadle, arises from the piano bench, delivers a line of dialogue at the precise moment needed, then hits the ivories again. Ms. Lupone finds time in a complex ensemble number, in which she’s already tooting the tuba, to clang a triangle held by another player.

“It’s very intricately worked out,” said Mr. Doyle, 52. “Nothing’s left to chance. You can see why none of them can ever leave the room. There’s no choice. They don’t really forget things, because they never stop working. It’s not like, ‘Oh, it’s not my scene.’ They’re constantly dependent on each other.”

Mr. Doyle, who has a mild manner and a face to match, has spent much of his career in the provinces of the UK, running theatres in Liverpool, Chester, and York. It was at Liverpool’s Everyman Theatre that he first armed his actors with bows and reeds, staging a production of “Candide.” “It did grow out of a world where, like any regional theatre in the UK, we had limited resources,” he said. “Here I am now in an environment that doesn’t have the same limitations, but I would still do it this way.”

His spare “Todd” has no fancy barber chair – his client’s deaths are signalled by the transferring of red liquid from one bucket to another; each character’s history and inner life is communicated through a towering shelf of bric-a-brac that, vine-like, climbs the back wall of the theatre. “Everything is suggested,” Mr. Doyle said. “Everything is symbolic.” The result is a production that, at times, casts that dazzling spell only associated with theater when it attains its purest form.

In England, John Doyle has developed an informal repertory company of actor musicians he turns to time and again. Here, his casting agent threw out a wide net over a four-month search. Not surprisingly, several members of the cast are making their Broadway debuts. Mr. Doyle claimed he was prepared to cast lead actors who didn’t play anything – Sweeney in the London production was instrument-free. But he got lucky.

“Because Michael plays the guitar we were able to use that in a couple numbers,” he said. “He has a limited amount of time when he can play. I didn’t know Patti played the tuba. She was so game. She didn’t want to stand out like, ‘Here’s the star who doesn’t play anything.’ Some people wouldn’t go there. But they offered things that they could do.”

Since the New York actors finger different instruments than those who performed the show in London, the show required completely new orchestrations. In fact, from casting on, a domino effect took place, causing Mr. Doyle to utterly restage the piece.

“When the orchestration changes, the physical use of the stage changes as well, because if you’re moving a violin around, it’s different from moving a flute around, and you have to put it down in different places and use it in different ways, which inevitably means that the whole jigsaw puzzle changes.”

Mr. Doyle will leave New York shortly after the November 3 Broadway opening of “Sweeney Todd” to restage his “Mack and Mabel” in England. After that, he’s been drafted to stage an “actor-musicianship” version of Sondheim’s “Company” in Cincinnati. But he will check on his Broadway children from time to time. “The family that is the show is terribly precious to me,” he said.

“I teased them that the time will come when they’ll sit in bars after the show discussing the tempi,” he said. “That’s exactly what’s already happened.”