NEW YORK » Hattie Morahan, who stars as Nora in the acclaimed production of "A Doll’s House" now at the Brooklyn Academy of Music, has won the kind of rave reviews any actor would envy.

But for about two minutes during each show, she has found herself upstaged by a performer who doesn’t have any lines and, in fact, doesn’t seem to be acting at all.

The performer appears about halfway through the first act, wriggling in a white dress and gray sheepskin vest, and a delighted murmur ripples through the crowd.

"It’s a real baby!" one audience member whispered during a recent matinee. There were more than a few audible "Awwww"s of the sort more typically elicited by viral YouTube videos than by dark Scandinavian psychodrama.

"I was surprised by my own response," Evan Lichtenstein, 29, of New York, said during intermission. "But then I just met a new baby last week" – a friend had become a parent – "so I guess I was primed to react."

Real infants are an extreme rarity on the stage, for all the reasons you might imagine, starting with rogue bodily fluids. And in the lobby at the Brooklyn Academy’s Harvey Theater, where the production runs through next Sunday, audiences have been buzzing about the sheer novelty, and about whether the unexpected cuteness-bomb raises the dramatic stakes of Henrik Ibsen’s classic drama about a bourgeois wife and mother who is jolted to a shocking decision, or takes theatergoers out of the play.

At the matinee, a group of ninth-graders on a field trip from the School of the Holy Child, a Roman Catholic girls’ school in Rye, N.Y., pronounced the infant "adorable." A doll, one student added, "would have been distracting."

But others were puzzled over the presence of the baby, which smiled beatifically through some enthusiastic cheek pinching by Morahan.

"It was unnecessary," said Elsie Kilvert, 75, of New York, adding, "The babies in my family would not have been so calm."

The role is shared by three infants, two boys and a girl, none of whom would comment. But Ida Lenert, the mother of the 8-month-old Liam, who appeared at the matinee, said that her son had taken to his newfound vocation.

"Liam likes the action, he’s pretty social," said Lenert, a Swedish-born cinematographers’ agent living in Brooklyn, who said she had heard about the role through a friend from her prenatal swimming group. "You never know how a little baby is going to react onstage. But he seems to get all focused and take his role seriously."

As with all actors, routine is important.

Sarah Tryfan, the stage manager for the Young Vic production, said by email that the babies spend only about an hour and a half in the theater. (They are paid $125 an appearance. The actors who play Nora’s two older children are paid higher Equity rates.) They get their own dressing room, where they nap, eat and play with Leda Hodgson, who plays the family’s nanny, before their brief cameo.

"We will never send a baby onstage if it is showing any signs of distress," Tryfan said.

A realistic doll is kept on hand, in case the baby has too much of a diva moment. There was a recent close call, she said, involving a baby who had recently learned to blow raspberries, but so far nothing like the night during the production’s earlier run in London’s West End when the baby started screaming five seconds before his entrance.

"I immediately sent the actors onstage with the doll and rushed the baby away into a corridor, where he projectile vomited all over his mother," she said. "Within seconds, he was completely happy again. It was a rather close escape."

Ibsen’s text does not specify the age of the family’s youngest child, Emmy, whom Nora calls "my sweet little baby doll." Photographs from the 1975 Broadway production, starring Liv Ullmann as Nora, show her romping with a little girl of about 5.

Carrie Cracknell, the current production’s director, said in an interview that she had wanted to use an infant – and a real one – to make the stakes of Nora’s decision to leave the family home, possibly forever, as vivid as possible.

"The door slamming has become this feminist moment of triumph, of breaking free of an oppressive marriage," she said. "But we also understand it as the beginning of uncertainty and heartbreak." Even today, she continued, "audiences still find the fact that Nora leaves her children very difficult."

Some plays involve babies in ways that are simply too charged for real infants. It’s hard, for example, to imagine anyone casting a real baby in Edward Bond’s "Saved" (1965), even if a doll were swapped in before the infamous scene in which a baby, temporarily abandoned by its mother, is stoned to death in a park.

Even in less fraught dramas, there remains the problem that babies don’t start, let alone stop, crying on cue. For the Labyrinth Theater Company’s 2013 production of "A Family for All Occasions" – Bob Glaudini’s play about a teenage mother who ultimately makes a Nora-like decision – the director, Philip Seymour Hoffman, experimented with a hyperrealistic doll, but ultimately went with an empty bundle of pink swaddling clothes equipped with a speaker that emitted recorded cries.

"The baby cried almost continuously over the dialogue" in the second act, the production’s props master, Matthew Frew, said recently. "It added to that feeling of burden, of the family being overwhelmed."

The baby in "A Doll’s House" goes home to bed long before the curtain call. But at the recent matinee, several in the audience gave Liam high marks for his performance, or rather lack of performance.

"It was miraculous," said Patti Matson of Manhattan. "The baby did exactly what it was supposed to do, which was nothing."

Jennifer Schuessler, New York Times