"A COMPLETELY amoral woman . . . scheming, childish . . . it's the most important role I've played in the last 10 or 15 years," says Angela Lansbury, primly tucking up one leg on the sofa of her Manhattan apartment. And then she smiles, rolling the next line over her tongue.

"I don't think anyone could honestly find 'Sweeney Todd' distasteful. We may slit a few throats, but we do it in rhythm."

Murder, cannibalism, rape, mayhem -- something for everyone, a comedy tonight? Well it's not everyone's piece of pie, maybe, but "Sweeney Todd" is without question the most successful bete noire musical since "Threepenny Opera."

When it opened on Broadway in March, 1979, Rex Reed called it "one of those rare, extraordinary and awesome spectacles like the Aurora Borealis." Newsday called it "a metaphor for a voracious society"; Women's Wear Daily described it as "a shattering experience."

What is this "awesome spectacle"? A Stephen Sondheim rendering of a onetime penny dreadful called "Sweeney Todd, the Demon Barber of Fleet Street," in which a 19th-century London barber, seeking bloody revenge against a crooked judge who sent him to prison in Australia in order to ravage his wife, slits the throats of numerous notables and drops them through the trap door to Mrs. Lovett's kitchen, where they become the main ingredients of cheap meat pies . . . Not quite the classic boy-meets-girl Broadway ticket.

But Lansbury, 55 and taking her pick of stage and screen roles spent a year on Broadway as the sanguine Nellie Lovett and is preparing for a 10 1/2-month return engagement on the road, beginning Friday with a month-long run at the Kennedy Center.So what's the attraction? Lovett herself -- the most delicious, malicious pragmatic madwoman in musical history. Next to this woman, Lady Macbeth mewls like Melanie Wilkes.

"I really had to work at justifying what she's doing," says Lansbury. "It's pretty hairy stuff. Very hairy in fact -- she's always picking hairs out of her pies."

Lansbury was recruited for "Sweeney Todd" by co-producer Richard Barr, a longtime friend of Lansbury and playwright Edward Albee's one-time producer. Lansbury had been in Ireland preparing to play Martha in "Who's Afraid of Virginia Woolf?" under Albee's direction when Barr cabled that "Sweeney Todd" was hers for the asking. She made Barr ask Albee to release her, then she flew back to New York to consult with director Harold Prince and composer-lyricist Sondheim.

"We had to tailor the part to my particular needs," says Lansbury, stretching her arms with a practiced casualness across the back of the sofa. "We had to make her a part worth my playing." In the original version, Mrs. Lovett was strictly a secondary character, well below star par, but in the hands of Lansbury and Sondheim she became "the pusher, the mastermind" of Sweeney Todd's revenge.

And well she might be. Lansbury, a native of London who made her film debut in the 1944 MGM classic "Gaslight" (receiving the first of three Oscar nominations), has a theatricalityto be reckoned with. Since her Broadway debut in 1957 in "Hotel Paradiso," she has picked up four Tony awards, and all for domineering, stage-center roles: Mama Rose in "Gypsy," the Madwoman of Chaillot in "Dear World," "Mame" and Nellie Lovett.

Of them all, Mame is still the favorite. "There's nobody quite like Mame," Lansbury says warmly. "I'm not like Mae, but I understand her mentality, her zest for life, her sincerity. And I could contribute to it."

Mrs. Lovett, on the other hand, was a stranger: "It's complete and total characterization. Mrs. Lovett bears no resemblance to me. Every piece of body movement and every piece of dance had to be coldly considered."

Sweeney Todd and Nellie the knife are as familiar in England as Punch and Judy. The 20th-century play by Christopher Bond was based on an old melodrama, "one of those plays that was always pulled out of the trunk by traveling companies," says Lansbury. "In fact, there was a man who made a career of playing Sweeney Todd. His name -- I don't know if he changed it to match -- was Todd Slaughter, and whenever he came out on state, the audience would boo and hiss, that sort of thing."

The story actually reaches back to 14th-century France, but took permanent hold in England with the 1846 "Demon Barber" serial by the author of "Varney the Vampyre." The current "Sweeney Todd" uses a book by Hugh Wheeler, although it is vitually all told in song.

"Out show is very Dickensian," says Lansbury, and in fact the grim and cavernous mechanical set -- a defunct Rhode Island iron foundry that was reassembled in the Uris Theater for $100,000 or so -- evokes like "Oliver Twist" the soul-chilling labor of the industrial revolution. (that foundry, says Lansbury, has become a virtual part of the structure of the Uris; the road set will be visually similar but tons lighter.)

"You can't help but feel sympathy and empathy for Sweeney Todd," says Lansbury. "He's caught in a web of circumstances, stripped of his wife and child and shipped away on trumped-up charges." After 15 years, Todd escapes his prison and returns to London only to find his wife missing and his daughter a ward of the lascivious judge. His only ally is his former neighbor Mrs. Lovett, the baker of mysterious meat pies.

"She's the one really who finds it expeditious to use the various bodies for pies," says Lansbury. "I mean, the economy's in shreds and there's not much ready meat available -- not many cats and dogs, either.

"Being an amoral woman, a good-hearted slut of the streets, she takes advantage of the main chance and thinks nothing of it. She's very practical-minded. Without her, Sweeney Todd would never have got it together. She pushes him, cajoles him.

"And she does it because she loves him, she always did. She dreams of a house by the seaside where they could retire with their money, occasionally bumping off the odd paying guest."

Lansbury plays this undaunted murderess with manic merriment, her hair bundled up in two horns, her eyes soot-circled and her painted face animated by a rictus sardonicus . "And always singing," she says. "Passionately singing, endlessly singing."

Sondheim's score is exhausting, as a matter of fact: rapid-fire and hilariously sly. "He killed for love and he loved to kill," lilts "The Ballad of Sweeney Todd," which meanders in and out of the action. In "Try the Priest," Todd and Mrs. Lovett compare the relative merits of particular ingredients: The trouble with poet Is how do you know it's decreased? Try the priest.

But the action is nonstop, and Lansbury, whose character is responsible for most of the comic continuity, is opting out on the Wednesday matinees (unwelcome news for subscription ticket holders), popping vitamin pills and cooking all her own food to ward off even the passing upset stomach.

Stage homicide is not a new habit for Lansbury. Her very first role -- a wonderful irony that hasn't occurred to her before -- was as another mass murderer, Bluebeard. "'Bluebeard and His Eight Wives,' that was it," she says, her face suddenly, uncalculatedly alight. "I was 10 or 11, in school in Hampstead, and I got to make all sorts of extraordinary faces." And she rubbers up her face like a small child on Halloween, all squint and sneer.

"My friends were hysterical in the back, byt I got quite a hand."

Lansbury and her husband-manager, Peter Shaw, became U.S. citizens in 1951. They own a condominium near Lincoln Center on Manhattan's West Side, and a house in Cork, Ireland. Their son Anthony is an actor and their daughter Dierdre lives in Italy.

Spliced into her stage productions, Lansbury has made more than 40 movies. She recently finished filming "The Mirror Crack'd," the first of three Agatha Christie mysteries in which Lansbury will revive the redoubtable Miss Marple.

"I like to make movies, but I much prefer the theater," she says, her faint British accent becoming slightly more noticeable. "Acting is my business, and I can't act without an audience. Movies are so intermittent: You shoot one sceen and then you sit around for 12 hours waiting to do another and I never know what to do in the meantime."

On the other hand, there seems to be something of a shortage of interesting theater these days. The Great White Way is riddled with revivals, and Lansbury is concerned about the thin crop of new authors and composers.

"We live in a time of such changes," she says, looking out over America's totem metropolis. "We can't glorify any phase of our society, or the way we are, and therefore people find it very hard to find inspiration.

"Look at the great Broadway shows: Lerner and Loewe, Rodgers and Hammerstein, Cole Porter -- they wrote in a time when you could look forward with joy in your heart, when the people you knew were honorable people, by and large. But now we can no longer be so optimistic."

What Broadway needs, Lansbury suspects, is a shot of some new style, like the one Sondheim is playing with in "Sweeney Todd," more fluid, more cinematic -- or perhaps something even more innovative. "I guess someone will show up," she says, shrugging. "They always have."

But even Sondheim, Lansbury and Prince have a revival up their sleeves. In 1982, the "Sweeney Todd" trio plan to launch a musical version of "Sunset Boulevard."

"We'll set it in the '80s and make the [Gloria Swanson] character a musical star of the '50s . . . Betty Grable, Rita Hayworth, Doris Day. It'll be a terrific part."

Complete with one more murder.