When Beverly Sills made her debut at the Metropolitan Opera last week, it turned, as it had to, into a free-for-all for her fans and the critics--an orgy of adulation. At 45, she's a superstar who has made it without the Met, which refused for years to recognize her and which now, with Rossini's The Siege of Corinth, has finally mounted for her the kind of production she deserves. Her fans paid up to $500 to see her on opening night and were in no mood for restraint or even courtesy: they cut off a singer in mid-phrase as soon as Sills stepped quietly on stage to give her a two-minute ovation.

Sills gave a dazzling performance of technical brilliance and dramatic depth. Sills the singer tossed off the intricately ornamented bel canto lines with fire and easy grace; her voice is a light silvery instrument that takes cadenzas at breakneck speed and makes them sparkle. Sills the actress managed to breathe life into the flat character of Pamira--the daughter of the governor of Corinth who is torn between love for her country and love for the Turk King Maometto, her father's enemy. Sills's Pamira was emotionally focused--a earess of Maometto's arm conveyed sexual delight, and one act later a subtly different touch of the sleeve of Neocle, the Greek warrior she's supposed to marry, indicated a dutiful, patriotic love without passion. The libretto is tedious, and often slightly ridiculous--several of the most dance-like arias have the most tragic texts, and Sills pulled off a real feat in making these passages sound agitated rather than light-hearted. If her voice now shows occasional signs of stram and her highest notes are slightly pinched, it makes no difference: Sills's special gift isn't perfection, but the perfect communication of her art.

The rest of the cast did her justice. Mezzo Shirley Verrett, in the travesti role of the warrior Neocle, revealed a stupefying coloratura technique and an enormous range of vocal textures. While Sills obviously paced herself carefully throughout the three acts. Verrett let loose in her first aria and blazed to the end at full capacity. She was gloriously confident, full of defiance and swagger, and poured out her ten-minute solo aria in the third act with the beauty that comes from inexhaustible strength. As it turned out, she had a mob of fans in the audience to rival Sills's; the Sills people were wildly adoring but the Verrett people were fierce--they had more to prove. Verrett's solo netted a five-minute ovation, rhythmic pounding on the floor, shouts of "Brava, diva!" and a shower of shredded programs from the top balcony. The Sills fans were wealtheir--they threw bouquets of roses from the first-tier boxes.

Justino Diaz, Sills's leading man, has some rarely combined talents for a basso; the lightness and flexibility that coloratura singing demands, and a natural sound so powerful that it cuts through the orchestra as clearly as the shriller upper voices. Like Sills, he does a lot in this production with a boring character. The passion he shows towards Pamira, especially in their love scene, is about as much as they ever allow on the Met stage.

This version of The Siege of Corinth, pieced together by conductor Thomas Schippers from 35 varying scores of the opera and 2800 pages of original manuscript, is seamless and vibrant, and adds a rare tragic work by Rossini to the stock of his popular comic operas. Schippers is apparently as good at sewing musical segments together as Rossini, who constantly borrowed from old operas to write new ones, and who was so cavalier about detail that if a page of his manuscript fell to the floor while he was composing, he'd write a new one from memory, being too obese to bend down comfortably to pick up the original.

For Sills, the Met debut is a major event in her career, but not in the way it would seem. She's already a superstar, so the Met's belated recognition of her is a slight anticlimax. This production of Siege, mounted in 1969 for the centennial of Rossini's death, was the vehicle for her debut at La Scala in Milan; even then she had already been acclaimed as one of the greatest coloratura sopranos of this century. The production was so successful that the Met bought it and signed its first contract with Sills--that bastion of complacent conservatism having taken six years to acknowledge what everyone else had known for a long time that Sills, an American, was as good as any of her European colleagues and better than most.

She auditioned for Rudolf Bing, the former general manager of the Met, early in her career, but he wasn't interested. So she joined the New York City Opera, made it big ten years later in Handel's Julius Caesar, and afterward turned down all the contracts Bing gingerly offered. No other singer has been able to do this; it is her legend, and she's proud of it. It became a David-and-Goliath myth which her fans loved and continually embellished, the most ardent even supposing that she refused contracts solely out of righteous indignation for not having been appreciated earlier. Recently she's become pretty candid about what was actually in those contracts: insultingly poor roles, old productions, or performance dates that Bing knew conflicted with her commitments to sing elsewhere--in one instance, her debut at Covent Garden.

Justino Diaz, her leading man in this opera, confirmed all the reasons she's been giving her interviewers lately about the origin of these offers tailored for refusal. "It could have been that they wanted her to say no; then they could have said. 'Well, we invited her but she refused.' It could have been a clash of personalities: maybe so-and-so didn't like her that much. Maybe they considered that she really was not that well known to deserve a new production in a really big role." He paused, and then added reflectively. "This happens to all singers, I don't care what your name is. There's always somebody who can't stand your voice. You never know what reasons these people have for acting the way they do."

Sills has stressed that she bears no grudges and is having the time of her life in this production, Diaz says, "She's very warm, has a terrific sense of humor, and is wonderful to work with." The first dress rehearsal, five days before the premiere, was chaotic--conductor Schippers was already exasperated, snapping angry commands at the musicians: Diaz, whose cape was falling off, was trying in vain not to trip on it; there were mistakes in blocking; an 'extra' kept dropping his spear with an audible clatter; and Sills handled it all by laughing. The more tired she got, the more often she sat down between arias to massage her back, the more she joked with Diaz and Verre,; a few times they missed their cues and had to whirl around and catch the line in mid phrase. At one point while Sills herself was running through an aria. Schippers barked out a directive--she thought he was going to stop so she started to speak. A second later she realized that the music was still going, caught her ascending scale where she had dropped it and carried it on up without missing a beat. As the notes rolled out she was shragging her shoulders, gesturing and mugging comically into the wings, making everyone laugh but Schippers, who never laughs. It seemed that she was clowning on purpose, to make the rehearsal less tedious--behavior entirely consistent with a personality so generous and secure that she was willing to share her debut with another female singer whose vocal abilities are as prodigious as her own.

Sills came to the Met secure in her stardom. For Diaz's career, the principal male role in Siege means something completely different. Ten years younger than Sills, he has been at the Met for 12 and yet his greatest role to date--that of Count Cenci in Ginastera's Beatrix Cenci--didn't take place at the Met: Beatrix Cenci opened the Kennedy Center in 1972 and ran for two years at the New York City Opera. The role of Maometto, which he sang at La Scala with Sills in 1969 for his own debut there, is another crucial triumph--his career is well-established internationally but still on the rise. As for the Met, he says, "My position here, of course, is not really steady. I'm not like a weekly artist--I go wherever the jobs are. I just keep travelling and wherever the right roles are, there I'll be."

It's only a minor problem for him that in the most popular opera repertoire, tenors tend on the superstar level to eclipse basses, who often find themselves stereotyped as priests, old men, court advisers, and evildoers. Diaz recognizes this--"I have no choice, I'm sort of stuck with my voice range. What can you do? The repertoire has already been written"--but doesn't think it will stop him. He laughs. "I still have an enormous variety of roles to play, and I unrealistically expect my voice to laugh forever!"

One of his favorite parts has been the title role in Mozart's Don Giovanni: the Italian equivalent of Don Juan who, having seduced over 1000 women before the opera starts, goes on to seduce a few more before he's packed off to hell. Diaz says that he tries to portray this character as a glorious rebel rather than as a rank scoundrel, adding. "From a chauvinistic point of view you could say, look at all the women he made happy! Apparently he was an irresistible character--a terrific lover, a wonderful human being, at least for those few minutes or hours or days which he chose to spend with these assorted ladies." Lounging comfortably in his dressing room, barechested, he lights a cigarette and chuckles. "A thousand women can't be wrong!"

Perhaps because he lacks the absolute security of a star like Sills, he is more temperamental and serious in rehearsal than she is. He has very definite ideas about how he wants to sing his lines and he's willing to fight for them. After rehearsing the third act-he stormed off stage, saying heatedly to Sills, "It's musically completely wrong for me. Why should I sing that way? It's ridiculous." He was tense in the dressing room as he explained. "A tempo is not right, which makes it uncomfortable. We'll have to iron it out because if not, well, what is the point of doing it?" Asked whether he has to fight with the conductor about that, he laughed exasperatedly. "Not fight, but you have to convince him that you need help." Has he ever had a big fight with a conductor over issues like this? "No, no, just little fights, little fights," he said, as he lit another cigarette and steamed agitatedly back and forth. Monday night, as his powerful voice rolled up to the fourth balcony, it looked as though he'd won.

Diaz stresses his belief that the Met's hiring practices are based firmly on talent, and he has reason to be pleased with his career there. In addition to opening the season next year in Siege, he will sing the title role in Mozart's The Marriage of Figaro in November. "I can now say yes or no to anything they want me to do. I don't have to take whatever they dish out. But I think that all of those problems have been ironed out very naturally and very cordially." Problems? Who said anything about problems? He's not saying; whatever they are, they're behind him.

Sills is contracted at the Met through 1977. Diaz will probably be there as long as he chooses to stay. Diaz's career is proof that the Met system has been basically a meritocracy in the past; Sills's debut now demonstrates that general manager Schuyler Chapin has made it a more perfect one.