Curled up on a straight-backed chair in her grim subterranean dressing room at the Kennedy Center, Audra McDonald is laying out her definition of “divas.”

“People who just whip up what they do,” she says. McDonald snaps her fingers for emphasis, a short pop that comes with the word “whip.” “They just tear it up, it’s so good. They tear it up.”

McDonald knows her divas. She’s the one, after all, who shares the Eisenhower Theater stage with Maria Callas for most of the second half of Master Class, Terrence McNally’s much-acclaimed new play. La divina, of course, is inhabited by Broadway legend Zoe Caldwell, herself no slouch in the grand excess department. The play’s premise is that Callas, preparing for a return from retirement, is conducting a series of master classes for young singers. Twentysomething McDonald plays Sharon, the cocky young soprano who strides onto the stage one afternoon in a drop-dead evening gown, ready to dazzle the invited audience with Lady Macbeth’s fiendishly difficult Letter Scene. Callas, of course, shreds her before she’s sung a note, and Sharon beats a hasty retreat, though she summons up the courage to return later. She spends the interval, we learn, “throwing up in the ladies’ room.”

“People think Terrence is taking it too far, that she was never that rough with those kids,” says McDonald, who’s listened to the recordings of the master classes that inspired McNally’s play. “Not at all.”

Callas’ ferocity wouldn’t stop McDonald, a Juilliard-trained singer, from jumping at the chance to sing for the diva if she were still alive. “I think I’ve got the nerve,” she says, and then she laughs, twisting a leg up to sit on it, yogalike. “I’ve got the nerve for a lot of things, but would I survive it? I don’t know. Because I’m good at passing out. I’d get there, I’d get dressed, I’d show up, and then I’d probably faint.”

Such predictions are not hyperbole. McDonald’s most public swoon came just a few months ago, during a recital of show tunes and classical standards in her home state of California. Before that, she passed out cold during her fifth and final audition for the role of Carrie Pipperidge in the 1994 Broadway revival of Carousel. She got back up and went on, though, and after she dusted herself off and sang through “When I Marry Mr. Snow” a second time, she got the part.

Some critics, the ones who couldn’t come to grips with what’s delicately termed “nontraditional casting,” howled their outrage. “A lot of people were real pissed off,” McDonald remembers, a hint of disgust edging into her voice. “There were the John Simons of this world, who just thought that there couldn’t be anything worse than casting a black person opposite a white person in a Rodgers and Hammerstein musical. Except maybe the Holocaust. They just thought it was so horrible.”

Luckily for McDonald, most of New York disagreed. A little over a year after her graduation from Juilliard’s voice program, while most of her classmates were still making the regional opera rounds, she won herself a Tony.

“I really am just now starting to wake up out of the haze of the last two years,” McDonald admits. “You know, like when trauma victims go into shock, they don’t feel a thing. Everything I had ever dreamed about having happen, happened. And I wasn’t really there—I just was kind of stepping back, going, “God, look, that’s happening to somebody.’ ”

This is not the way most aspiring divas start their careers, and success in musical theater has done in any number of opera singers before now. In the days when baritones made bankable Hollywood stars, Robert Merrill was blackballed by the Met’s Rudolf Bing because he was so crass as to be commercial; more recently, Julia Migenes found herself out of favor when she wouldn’t leave her nightclub gigs behind after she hit the big time. McDonald refuses to worry, though.

“I’m still young, and so I haven’t been pigeonholed yet,” she observes. “I still have time to start on an operatic career, should I want one. But I think if I just choose not to let people make the decision for me, they can’t pin me down into one thing. People are crossing over; look at Dawn Upshaw.” Besides, if McDonald had passed on Carousel, she wouldn’t have had those backstage chats with Shirley Verrett, one of the great sopranos of the ’70s and ’80s, and a formidable Lady Macbeth in her day. Verrett played Julie Jordan’s cousin Nettie Fowler, and she was the one McDonald went to see when she was thinking about auditioning for Master Class.

“I told Shirley I was auditioning for it, and I said, “I know you’ve sung Lady Macbeth, this is the aria.’ And she said, “Well, you better have some high C’s, missy.’ And I thought, “C’s? Plural?’ ”

Daunted by the prospect, McDonald actually canceled her audition. But the Master Class producers asked her to reconsider, and after a little coaching from Verrett, she decided to tackle the aria. “It’s just that it’s all over the place,” McDonald explains. “It’s not necessarily that it’s high, touch wood. That’s OK. It’s that you go from high C all the way down to your chest again; it’s just up and down and up and down, and it’s all like this,” she says, illustrating with a dramatic, tense gesture. “There’s no time in the entire aria when Lady Macbeth calms down and reflects on anything she’s said. She doesn’t, she’s just like, do it, do it, do it. And you’ve got Madame Callas behind you screaming for more passion. I just didn’t want to make a big fool of myself.”

Of course, the play’s construction gives McDonald an out if she ever has problems with the piece. Sharon’s meant to be overreaching herself with her choice of aria, after all; Callas says as much at the end of her lesson. And despite McDonald’s diligence (and her big, lush voice), some theatergoers are hard pressed to remember anything but Zoe Caldwell, who’s like a great black hole drawing all the house’s light and energy toward her. All the better, laughs McDonald. “It takes some of the pressure off me.”