Each night, when Alison Pill slips into the lead role of Theresa Rebeck’s new Broadway play, Mauritius, she must endure what can only be described as a theatrical gauntlet. She gets tossed about the stage like a three-ounce rag doll. She has to hold her own against F. Murray Abraham—the F. Murray Abraham—and newer delights like Bobby Cannavale. She has to cry on demand, play fragile and gutsy, and generally serve as the emotional anchor of this Mamet-style play about wounded souls—and stamp-collecting.

And yet, ask the 21-year-old actress how she is surviving, and her pale-moon face begins to glow like a nightlight.

“It’s gotten more and more fun, and more and more empowering to be able to have the guts that this character has—to even pretend for a second that somebody has that amount of balls,” she said unleashing one of many long, galloping bursts of laughter (seriously, some lasted seven or eight seconds).

It was a late September afternoon, less than two weeks before the play’s Oct. 4 opening, and Ms. Pill was sitting, slender and unnoticed, at one of the outdoor banquettes of her favorite East Village coffee joint, MUD. Dressed in jeans and a black tank top, with jittery fingers and poised, button features, she seemed to hover somewhere between frail, vigorous and jaunty.

“She’s the most hopeful character I’ve ever played,” Ms. Pill continued. “It’s so glorious.”

Ms. Pill was not being facetious. While her character, Jackie, is no kitten—she is, in fact, a “damaged” kid who is desperate to escape her crappy past by selling off a contested family stamp collection—she’s a remarkably buoyant creature compared to some of the other roles the actress has tackled.

Ms. Pill, you see, has already been around the crazy-character block several times during her three and a half years in New York City. She has played a scrappy girl terrorist in Martin McDonagh’s The Lieutenant of Inishmore—a play that marked her Broadway debut and earned her a Tony nomination for featured actress. She has played the tormented victim of a Lolita-style love affair in Blackbird, an off-Broadway play for which she won all kinds of critical praise and, yes, more nominations (that time from the Outer Critics Circle and Drama League). She has played broken girlfriends and clinically depressed teens and now, with Mauritius, a complicated young woman who also happens to be her first starring Broadway role.

Taken together, it’s enough to have earned her the title, unusual at any age, of genuine stage actress—or, even better, theatrical throwback.

“I have a feeling she was born to this, it’s just one of those things,” said Mr. Abraham, who plays opposite her in Mauritius as a wealthy and thuggish stamp collector—yes, a thuggish stamp collector—intent on prying the stamps from Ms. Pill’s character’s possession. “She’s got the potential to do the Great Roles, and I don’t think I run across that very often—or people who are interested in doing it. It’s what I believe an actor should be doing.”

This, however, is not what most actors are doing, particularly the young ones. In these pantiless times, most pretty lithe things with a spark of talent (or just the delusion of it) hightail it to Hollywood as quickly as they can, glancing toward the stage only when they need a quick credibility fix. The results are often less than happy.

But Ms. Pill, who doesn’t drive and insisted she hates L.A. (“I get honked at when I try to walk down the street!”), has embraced a rather different approach to the whole acting game.

She came to New York in early 2004. She settled in the East Village—first in an apartment with two random British dudes, then in her own place. She made some friends, learned to dodge rats, played foosball. She began auditioning. And steadily, if not slowly, she climbed her way from off-off-Broadway to off-Broadway, to the Great White Way itself. (Can people really use that expression anymore?)

Of course, it probably didn’t hurt that she had built a steady career as a kid actor during her teen years in Toronto. And, to be sure, she has done her share of pixilated popcorn fare, appearing in movies like Confessions of a Teenage Drama Queen as Lindsay Lohan’s dorky sidekick and Pieces of April as Katie Holmes’ prissy-perfect younger sister. (When asked what it was like working with these two tabloid honeys, she said, with an indulgent smile, “You know, I’m just going to give you the ‘They’re both great.’ And, uh … both very sweet.”) At the end of October, she will shine down from movie screens across the nation as Steve Carell’s daughter in Dan in Real Life.

Still, for all this, one gets the sense that movies are not where Ms. Pill’s big passion lies. They are certainly “fun.” They definitely help pay the bills. And they allow an actor to “focus more specifically, which is nice,” she said. But it’s the stage that makes Ms. Pill misty, that turned her from a wisecracking everykid into a dewy thespian as she spoke to The Observer, her eyebrows knitted so tight they quivered.

“I mean, the writing in plays is typically just much better than most other writing, or at least more fun to play, and you get to play with language instead of just, Duh,” she said, unlocking her brows and cocking her head like a latter-day Tara Reid. “But if it’s natural dialogue, there’s a poetry and a rhythm that you yourself are finding. And there’s something really interesting about that.”

If Ms. Pill had not become an actress, she said, she probably would have liked to try her hand at academia. What kind? “Physics. Quantum mechanics. Ya know,” she said with a cool swagger, before dissolving into laughs. And then, more seriously: “I would have liked to have tried to get more into politics, I think. I’d also sort of dreamed of a degree in English too.”

Ah, well. English and politics would have been nice. But the truth is, Ms. Pill has been an acting junkie since almost, well, the beginning.

As Ms. Pill described it, she first got bit by the performing bug when she was 10 or 11. It all began innocently enough, after a “guy who worked at CBC Radio” in Toronto heard her narrate a show for her children’s choir (she also did ballet at the National Ballet School, and retains that telltale posture and willowy figure) and asked her to read for some books on tape he was producing. “I was like, O.K., sure, I can read,” she said, mocking her clueless child self. But soon she was pestering her mother for head shots, an agent, auditions. When she finally landed a job as an extra in Kung Fu: The Legend Continues, she was a “goner,” she said.

“I had the best time,” she recalled. “I fell so hard in love with everything about it.”

Ms. Pill made her first exploratory foray into New York’s theater world toward the end of her senior year in high school. By that point, she had already racked up a sizable list of TV-movie credits as well as Pieces of April (which was set, perhaps not incidentally, in the East Village). Live theater, however, was something completely new.

“I had no idea what I was doing,” she said bluntly of her first audition for a role in the New Georges theater company’s off-off-Broadway production of None of the Above. “I sort of might have said that I had done theater in Toronto, when in fact it was in a church basement, and it was like a camp thing. But I was like, ‘Oh, I am such a pro, you have no idea,’” she said, affecting the dripping accent of a 40-year-old diva.

The ruse seemed to work, or maybe she just had a really good audition, because Ms. Pill got the part, after which came other auditions and other parts. Sometimes the roles fell into place easily. Other times she had to beg—or simply dress up as a boy, take pictures of herself and send them to the director—as she did after auditioning for the Lars von Trier-Thomas Vinterberg collaboration Dear Wendy.

“I wrote an insane letter, and I just begged him to be part of the movie,” she confessed.

Then there were the times when, despite a sizzling audition, she simply didn’t land the part.

“We first saw Alison Pill when she came into audition for Doubt for the role of the Sister James, and she blew us away,” said Mandy Greenfield, associate artistic director of production for the Manhattan Theatre Club, which produced Doubt and Blackbird and is now producing Mauritius. “We were all left with the sensation that we’d just seen someone incredibly special.”

For all that—the sensations, the tingles—Ms. Pill was not cast in Doubt. (Alas, the cruelty of casting directors!) But over the next few years she would work fairly consistently, doing fine if not swell, until she hit the lovely, lucky streak she’s been riding for the past 18 or so months.

The streak began in the spring of 2006, when she made her Broadway debut as Mairead in The Lieutenant of Inishmore. Inishmore led to her first Tony nod, which, despite ending sans trophy, did culminate in a good party and nice dress (though she confessed that she nearly wound up in a terrifying sequined number that made her look “like a choir director from Russia performing at Carnegie Hall”). From there she went on to audition for another Manhattan Theatre Club play, the twisted sexual abuse saga Blackbird, and this time landed the part (opposite Jeff Daniels, no less). “She completely owned that Blackbird text,” recalled Ms. Greenfield. Her lead role in Mauritius followed from that.

THE STORY OF Mauritius is not a warm and fuzzy one. Directed by Broadway veteran Doug Hughes, it follows five rather ruthless characters as they try desperately—and sometimes comically—to cheat, scam and even strangle each other as they all vie for a potentially valuable, but potentially counterfeit, pair of stamps. It’s philately as a blood sport.

Ms. Pill’s character, Jackie, sits right at the beating center of the action. A sullen yet feisty post-teenager (Ms. Pill said that she and Mr. Hughes had decided the character is around 22), Jackie is the one who sets the story in motion, and it is her quest to sell off the stamps—and win herself a new life—that forms the arc of the play. Along the way, she gets knocked around, fights back, falls down, gets back up, falls again. For Ms. Pill, it translates into one long evening of fighting for respect—onstage, that is.

Offstage, it’s a different matter. As she described it, the reality of life behind the curtain resembles something far closer to the peppy fabulousness of theater camp than the dysfunctional slyness of Mauritius. The actors hang out together. They support each other. They sing karaoke, drink champagne, and even do each other’s makeup—or at least, Katie Finneran, the Tony-winning actress who plays Ms. Pill’s half-sister in the play, helps her paint on her eyebrows because “apparently I’m not good at painting on eyebrows,” Ms. Pill said.

And though Ms. Pill is a good 40 years younger than some of her castmates, and though she was certainly intimidated by one or two at first—uh, Mr. Abraham—she now finds herself sharing regular, gushy mutual lovefests with her fellow Mauritians.

“We all leave the stage and we all have a little group hug after curtain call. We go, ‘Mauuuuuuuritius!’” Ms. Pill confessed, before unleashing one of her operatic guffaws. “We’re all such theater geeks. It’s kind of lovely.”

Of course, hugs and karaoke don’t necessarily translate into reviews, and at some point—quite soon, in fact—the actors will have to contend with either the kind or harsh words of the critics. But when Ms. Pill was asked what she hopes for Mauritius, she spoke only in terms of the play. “I want it to loosen up and for us to really just sink into it and accept the fact that we get to take an audience through a really good story that is satisfying,” she said.

As for what her co-stars want for her, it’s pretty straightforward.

“Keep doing theater,” Mr. Abraham advised in his perfectly enunciated baritone. And then, without being asked, he volunteered, “I’d like to work with her again.”