Anna Netrebko in "Anna Bolena." (Photo: Ken Howard / Metropolitan Opera)

One night in London in 1734, two opera stars ended up on the same stage. Senesino played the part of an angry tyrant, Farinelli a hero in chains. The two were bitter rivals, but, so the story goes, when Farinelli sang his melting opening aria, “he so softened the obdurate heart of his oppressor that Senesino, quite forgetting his stage character, ran to Farinelli and embraced him, much to the surprise of the audience.”

Senesino, we would say, broke character.

Such an irredeemably tacky breach of narrative decorum is rare in opera today. That was what was so remarkable about what happened on opening night at the Metropolitan Opera last Monday.

The soprano Anna Netrebko was singing the fiendishly difficult title role in Donizetti’s Anna Bolena. The final scene began with her achingly beautiful rendition of the aria “Al dolce guidami,” its final note slowly diminishing to nothing.

The audience erupted in cheers that went on far longer than is usual at the Met these days. Ms. Netrebko, who had ended the aria gazing upward, suddenly gave a wide smile, driving the audience to even greater applause.

The critics were not amused.

“Netrebko does not worry too much about staying in character,” Anne Midgette wrote in The Washington Post the next day, comparing Ms. Netrebko unfavorably to Maria Callas. Anthony Tommasini was more charitable, writing in The New York Times that “Ms. Netrebko seemed to break character and smile a couple of times” but adding that “her look could have been taken as appropriate to the dramatic moment.”

Heidi Waleson in The Wall Street Journal: “And when the audience greeted her reverie with applause, she even broke character and grinned.” Martin Bernheimer in the Financial Times: “Netrebko enjoyed predictable ovations, and acknowledged some of them mid-scene with a ravishing smile.”

Why did everyone get so upset about a little smile? “It was near the end of opening night, in a new role, and it was going really well,” Ken Benson, a respected artist manager and teacher told The Observer. “It wasn’t an ordinary moment. My take is that she was saying, ‘We’re almost there, kids! We made it!’”

And after all, even if it’s gone out of style, there is a long tradition of operatic character-breaking.

In a 1976 Met performance of Puccini’s La bohème, the soprano Montserrat Caballé was sitting on stage during Luciano Pavarotti’s first-act aria. “He sang the aria and she was sitting in her chair,” Mr. Benson recalled, “and when he finished she joined the applause.”

For her farewell to the Met in 1985, Leontyne Price sang Aida, one of her signature roles. About five minutes into the epic ovation after her aria “O patria mia,” she dropped to her knees with emotion. The tenor Salvatore Licitra, who recently died of injuries sustained in a motor scooter accident, did a “whew” gesture, brushing his hand over his forehead, after nailing the opening aria in his surprise Met debut in Puccini’s Tosca in 2002.

“What was always a big applause moment was ‘Abscheulicher!’ in Beethoven’sFidelio,” Mr. Benson said. “I remember the only time Christa Ludwig sang it at the Met. It was the peak of her success in New York. She sang the aria, and I’ll never forget this: she raised her hands over her head like a prizefighter. It was a shared moment of triumph.”

But breaking character is by no means always about celebration or relief: Mr. Bernheimer, the Financial Times critic, remembered the tenor Jon Vickers shouting at the audience, “Shut up with your damn coughing!” during a 1974 performance of Wagner’s Tristan und Isolde. Maria Callas, for all her storied focus, broke character on several occasions, including once in Anna Bolena itself.

To be fair, being Callas, she didn’t so much break character as assimilate Anne Boleyn’s drama into her own. Callas had returned to Milan in 1958 after a scandal and was received coolly throughout the first act. At the act’s finale she rushed to the edge of the stage, as the costume designer Piero Tosi once remembered, “spitting her lines directly at the audience”: “Giudici? Ad Anna? Guidici?” (“Judges? For Anna? Judges?”).

“You could not dream what she did,” Mr. Tosi said. “It was a show within a show.”

James Jorden, the opera critic for the New York Post, shared a performance of Verdi’s Il Trovatore captured on YouTube, in which the mezzo-soprano Fiorenza Cossotto sings the wrenching narrative “Condotta ell’era in ceppi” with utter focus, ending with her hands clutching her head and her eyes wild. That’s when the real performance begins. She stays in character for a full 40 seconds before her hands drop and her eyes cloud with tears. She begins to weep. She crosses her hands over her chest and wipes her eyes. The ovation surges. She smiles and lifts her eyes to heaven, the classic character-breaking move.

In another clip, Ms. Cossotto and—again!—Montserrat Caballé end a duet from Bellini’s Norma at arm’s length from each other. The applause is so intense that eventually Ms. Cossotto pulls Ms. Caballé in for a hug. The video director had the right idea: as the ovation continues, he superimposes the embracing singers over the footage of the audience clapping and shouting.

The clip sums up what was once opera’s special quality: the intimate, charged relationship between singers and audiences, a relationship that these moments of breaking character acknowledge and celebrate with unique power. With its stylization and its larger-than-life emotions, opera has never been about unbroken narrative or cinematic realism. It is about going in and out of the drama, in and out of realism.

For much of opera’s 400-year history, star singers would travel with their own costumes and their own arias, which they would insert into any work in which they were appearing, regardless of the dramatic context. The character break is the distant descendent, or perhaps the residue, of such indulgences. While all of this may seem absurd and risible to us, the audiences of the past weren’t less connected to the drama than we are today, with our silences at diva entrances that used to be greeted with ovations or our polite applause after arias. If anything, they were more passionate, more attuned. Opera meant more to them; its drama was more vivid.

Then something began to shift. The soprano Mirella Freni had not appeared at the Met in 15 years when she returned in Verdi’s Don Carlo in 1983. “Don Carlodoesn’t give a big opportunity for the soprano until the end,” Mr. Benson said. “So after ‘Tu che le vanità’ she got a huge ovation. In a very dignified way she knelt on one knee. And she was pretty widely chastised for that in the papers. Speight Jenkins took her to task for it in the New York Post.”

Had the AIDS epidemic killed off opera’s most ardent, knowledgeable fans? Or was it that, by late 1970s and early ’80s, long-standing European trends—a focus on the director’s role and an emphasis on cohesive dramatic values—had reached America? Whatever the reason, the break from character began to be regarded as less celebratory than selfish. “I despise the character-breaking syndrome, period,” Mr. Bernheimer wrote in an email.

These days, as we worry about opera’s future, we have bet our money on its viability as the same kind of drama as the more popular forms of theater, film and television, with the same kinds of narrative rules. But audiences for those forms are largely passive; they don’t have the opera audience’s unpredictable, give-and-take relationship with singers. Opera may, in fact, have more in common with sporting events—a narrative that is stop-and-start yet remains coherent; an intense connection between performer and crowd—than it does with other performing arts.

That was what was so moving about Ms. Netrebko’s smile. The illusion shared by an operatic audience is strengthened, not weakened, by its occasional collapse. Opera is drama, yes—thanks to people like Callas, those values were restored. But it is also sheer performance, a quality that is allowed to subvert that drama from time to time. To be serious, opera has to be fun.

“I totally understood the whole moment,” Mr. Benson said. “I wish there was more of a bond between the stage and the audience today.”

That bond, though, is still there, and it’s still strong. On Saturday, the boyish tenor Javier Camarena made his Met debut as Count Almaviva in Rossini’s Barber of Seville. Near the end of the opera, the count has a dazzlingly virtuosic aria that ends with a big high B-flat. The audience went wild, just as it had for Ms. Netrebko. Mr. Camarena stood and took it all in, and then he made a little bow, mouthing, “Thank you, thank you.” Nothing could have seemed more gracious or appropriate. Then the show went on.

As it happens, Ms. Netrebko was in the house. At intermission The Observer went over and asked her what had happened at the end of “Al dolce guidami.”

“Well, I don’t know, it was just a smile to the conductor,” she said. “It was kind of like, ‘O.K., we did it. It was good. The audience is applauding.’ It’s a beautiful moment to stay on the stage of the Met and receive all this applause, you know? It’s really a great moment. I think if it’s done just a little bit, without going too far, it’s O.K.”

“I didn’t do it last night,” she went on with a laugh, “because I was criticized for breaking character. I smiled a little bit, but not that much. I reduced my smile.”

editorial@observer.com