Three parts revelatory to one part awkward, John Doyle’s radical reimagining of “Sweeney Todd” at the Eugene O’Neill Theatre is far too adventurous to be entirely successful. No piece as screw-tighteningly effective as Stephen Sondheim and Hugh Wheeler’s 1979 Grand Guignol masterpiece of murder and cannibalism could withstand a top-to-bottom revamp like Mr. Doyle’s without shedding a few valuable pieces. But for every rough edge that gets sanded down, several others spring into unsettling relief.

What was originally conceived as a Brechtian epic has become a penny dreadful with on-stage instruments, a bare minimum of performers, and (literally) buckets of blood. But the smaller scale is not the big news here: Teeny “Sweeneys” have happened before and will happen again. Mr. Doyle’s main innovation is his use of the extraordinarily protean cast – which just happens to be the orchestra as well.

When she’s not creating a memorably carnal Mrs.Lovett,Patti Lupone is shifting between percussion and an enormous tuba. Lauren Molina and Ben Magnuson, terrific as the two ingenues, express their emotions as much through their soaring cello melodies as through their singing. Other cast members do similar double and triple duty, accompanying themselves and others as they perform Sondheim’s murderously difficult score – by memory, no less.

It would have been reasonable to expect the singing and acting to suffer as a result. And perhaps savvier musicians could find fault with the cast’s instrumental skills, but to this set of ears, the caliber of across-the-board talent is impeccable.The evening’s unsung hero is Sarah Travis, who has taken Jonathan Tunick’s marvelous original orchestrations and comfortably refitted them to the 10 (or fewer, depending on the scene’s requirements) actors.

Sure, a few dozen extra voices and instruments would add some thunder to some of Mr. Sondheim’s climaxes, but the production’s creepy intimacy would be compromised. By condensing the personnel requirements, Mr. Doyle has eschewed the memorable sprawl of original director Harold Prince for a claustrophobic,febrile world entirely of its inhabitants’ making.(This concept is also, let’s be frank, cheaper to produce. Sondheim revivals tend to do better with the critics than at the box office.)

Even with the villainous Judge Turpin’s optional self-flagellation number included, Mr. Doyle has knocked almost a half hour out of “Sweeney.” Mr. Wheeler’s libretto and Mr. Sondheim’s ominous underscoring have been pruned away ruthlessly, nearly every song has lost its “button” (the musical tag that cues the audience to applaud), and the lack of major props and sets allows the actors to change locations on a dime.

The spare set – Mr. Doyle also designed the production – requires a bold, impressionistic staging that defiantly resists Broadway’s penchant for spectacle. Sweeney has traditionally dispatched his victims with a razor that sends blood flying from the slashed throats. Mr. Doyle goes simpler and nastier: Upon each successful slash, Richard G. Jones’s lights bathe the stage in a deep red and a cast member methodically transfers a considerable amount of stage blood from one bucket into another.It’s undeniably “fake”and utterly effective. As the bodies pile up, the sound of that bucket filling closer and closer to the brim grows increasingly discomfiting.

Like many of Mr. Doyle’s innovations, however, the effect has its drawbacks. Several twists in “Sweeney” in volve close calls, as various good and bad guys wriggle free of Sweeney’s grasp just in the nick of time. An entire song, “Pretty Women,” hinges on this ebbing and flowing of tension. But the buckets only come out after a successful kill. The added level of stage business gives the audience too many clues – no bucket, no killing on the way.

Several other directorial conceits work beautifully until they suddenly don’t work at all. The use of a stepladder to denote an upper level is inconsistent and muddled, and while Mr. Doyle makes smart use of an unadorned black coffin, his design hits a sizable snag early in Act II, when Sweeney’s barber’s chair is replaced with a small white coffin.It’s meant to signify some sort of idealized representation of Johanna, the child torn from Sweeney 15 years earlier – but he knows Johanna is still alive, so why a coffin? As he lugs this little white coffin around for much of Act II, the sensation of a concept dragging a story behind it is hard to shake.

This occasional conceptual murk is offset, however, by the clarity of the vocal performances. The lyrics are clearer than for any Broadway show I’ve heard in years, let alone one with as many complicated parts being sung simultaneously, a testament to Dan Moses Schreier’s crystalline sound design and the superbly disciplined cast. Nearly half the actors are making their Broadway debuts; each one deserves many return appearances. And rela tive newcomers Alexander Gemignani (John Hinckley to Mr. Cerveris’s John Wilkes Booth in last year’s “Assassins” revival) and Manoel Felciano, playing the villainous Beadle Bamford and the innocent Tobias, further cement what promise to be two memorable Broadway careers.

Among the more seasoned performers, the cadaverous Michael Cerveris gives an intelligent but strangely unmenacing take on Sweeney. His Giorgio in the 2002 Kennedy Center “Passion” was an epiphany, but Mr. Cerveris seemed vocally underpowered at times in “Assassins,” and for all his skill at interpreting Mr. Sondheim’s lyrics, he resists giving songs like “The Barber and His Wife” and “Epiphany” the emotional zeal that they all but demand.The latter piece ends with Sweeney singing, “I’m alive at last / And I’m full of joy!” Cerveris’s Sweeney is unquestionably alive, but he has far more murk and sorrow inside him than joy or even, it seems, pure rage. It’s a crafty, empathic performance, and I wanted more.

Ms. Travis’s orchestrations set much of the early action in a spare, wheezing harmonic world evocative of Kurt Weill early on. The formidable Ms. Lupone slides into this world with surprising ease, channeling Weill’s muse Lotte Lenya with a sedate “Worst Pies in London” and painting a soberingly melancholy picture with “Poor Thing,” her recounting of the fate of Sweeney’s wife. But as Lovett’s romantic and en trepreneurial prospects perk up, she takes on an increasingly brassy, almost sultry tone.

Far less batty than Angela Lansbury and other predecessors, Ms. Lupone turns Lovett into a woman whose ruthless survival instinct hides crucial psychological blind spots. There’s a moment near the end when Lovett, after receiving a desultory kiss from the preoccupied Sweeney, says, “That was lovely.” I’ve seen more than a half dozen productions of “Sweeney,” and that line has always been delivered sarcastically, getting a laugh with a welltimed roll of the eyes. Lupone is dead serious: This Lovett is getting her needs met, which makes their relationship even more perverse.

Uncertainty and ambivalence, the motors behind most Sondheim musicals, are in scarce supply here. The demon barber of Fleet Street is here to make you squirm, not think. By doing so much (occasionally too much) thinking for us, Mr. Doyle plunges us into an entirely new “Sweeney Todd.” I readily admit that I look forward to the next time dozens of singers and musicians give life to the best score Broadway has heard in almost 50 years. But when I hear the lines “Sweeney heard music that nobody heard,” the spare, chilling music from Mr. Doyle’s virtuosic production is the music I will hear.

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