Actress Margo Seibert sits for a portrait in her dressing room at the Winter Garden Theater in New York, N.Y. (Jesse Dittmar/For The Washington Post)

It’s the oldest story on Broadway. And still the most irresistible.

A young actress, seemingly out of nowhere — okay, American University by way of Howard County and Glenelg High School — walks into an audition (well, actually, nine rounds of auditions) and earns herself that proverbial role of a lifetime, in one of the most buzzed-about new musicals of the season.

You’ve probably never heard of Margo Seibert. That’s all right. Neither had anyone else involved in “Rocky,” the $16.5 million musical based on the Oscar-winning movie and with a score by the creators of “Ragtime” that began previews Thursday at the Winter Garden Theatre. Such an unknown was Seibert, raised by a vigilant single mom who told her to major in international relations just to be safe, that even after rehearsals started, the show’s composer didn’t know how to pronounce her name. (It’s Sy-bert.)

The story of Seibert’s casting as Adrian — the painfully shy pet-store worker wooed and won by Rocky Balboa, the boxing hack turned world champ — is the one that has forever filled the fantasies of acting hopefuls. From the Pacific Northwest to the Corn Belt, from the prairie to the suburbs of Washington, young people armed with head shots and regional credits in Shakespeare and Rodgers and Hammerstein have from time immemorial migrated to cheap digs in New York, in the conviction that if lightning can strike, it might strike them. And even if it doesn’t, that there will always be a storefront off-off-Broadway somewhere, in which a director will be looking for one more promising actor.

“Very often people tell me, ‘Oh, I’m thinking of moving to New York,’ and I think, ‘Oh, jeez, that’s too bad,’ ” said Aaron Posner, a sought-after stage director and playwright based in Washington who cast Seibert in two of his shows at the Folger Theatre. “ ‘You’re getting work here, and it’s just so hard there.’ But when Margo told me she was thinking of moving to New York, I thought: ‘That’s going to be hard — but that’s a good idea!’ ”

Actress Margo Seibert (Jesse Dittmar/For The Washington Post)

That autograph-seekers crowding around the stage door of the storied Winter Garden may soon be shouting “Yo, Margo!” seems to bear out Posner’s intuition.

For Seibert, a vivacious 29-year-old, the smile that fortune has beamed in her direction is still being processed. “It just seems surreal,” she said the other day, sitting in the balcony of the Winter Garden, previously home to “Mamma Mia!” and “Cats,” during a break in “Rocky” rehearsals. “Everybody has been so excited and celebratory on my behalf. So now that the opportunity is here, I have to be brave enough to open the door and walk through.”

With music by Stephen Flaherty, lyrics by Lynn Ahrens and a book by Thomas Meehan, “Rocky” is another in a long parade of new Broadway musicals that are being lifted directly from brand-name movies, in hopes that popular and familiar properties make $143 orchestra tickets easier to sell. (Opening against “Rocky” this spring is the Broadway version of Woody Allen’s “Bullets Over Broadway” and a musicalized version of “The Bridges of Madison County.”) So in “Rocky’s” case, especially, the title is the star, easing the pressure on the production to fill the demanding leading roles with market-tested celebrities.

Rest assured that the selection of Seibert and Andy Karl — another Maryland native, who is playing the prizefighting part that in 1976 propelled Sylvester Stallone to fame — were made without much thought to the size of their fan bases.

It was, in fact, a special aura of honesty and accessibility that made Seibert stand out in the grueling, seven-months-long casting process. “Every time we had a round of auditions with Margo, at the end of the day, I would write on my little casting sheet: ‘Feels authentic, this is the kind of person I’d like to see in the show,’ ” said Alex Timbers, “Rocky’s” director. “I loved her from the beginning and, beyond her acting, which felt to me incredibly strong, and her singing, which was, too, it was that she looks like a human being.

“That sounds strange,” he added, “but what I saw with Margo was that this is the kind of girl that I can see walking around Greenpoint. And I think that’s going to be meaningful for audiences when they come in.”

That gritty and increasingly urbane Brooklyn neighborhood is where Seibert settled in sometime after moving in 2010 to New York from Washington, where she’d spent much of her 20s, performing small and mid-size roles in plays and musicals at places such as Folger, Signature Theatre, Shakespeare Theatre and Imagination Stage. The circumstances of her being told she would be making her Broadway debut seemed a perfect Adrian moment, too: She was at a babysitting job in Brooklyn.

“Alex gave me a call and said, ‘You’re going to get the offer officially. We all believe in you,’ ” Seibert said. Of course, she quickly dialed her mother, Debbie, an administrator in the Johns Hopkins University Applied Physics Laboratory in Laurel. (Her dad, Willis Seibert, a sometimes musician remarried and living in New England, was thrilled for her, too.) Despite her natural exuberance, the actress couldn’t whoop it up.

“The baby was sleeping,” she said.

Her mother says she felt that mixture of delight and relief known to every parent who waits for the world to finally understand what she knew all along. “Right before this, there was a moment where I started to think, ‘Is this ever going to happen?’ ” Debbie Seibert said by phone. “It’s a tough life. As a mother, you want to ask, ‘Are you sure you still want to live this way?’ We struggled financially, and I guess I didn’t want to see her go through that. And of course — she picked a field where exactly that happened.”

For every Margo Seibert, there are thousands of artists of every stripe who never get that call, and go off on other trajectories. Even for those who do, the triumphant moment can be fleeting. But let’s suspend those realities for the time being and focus on Seibert’s achievement, a Cinderella story if there ever was one, the kind that might have been cooked up in the amalgamated studios of Disney and LinkedIn.

She stayed in Washington after graduating from American in 2005 — heeding her mother’s advice, she majored in international relations instead of musical theater — and moved around the city, securing one acting gig after another. Having sung as a teen at Toby’s Dinner Theatre in Columbia, she gravitated to musicals, appearing over the years in “Reefer Madness” at Studio Theatre, “The Boy Detective Fails” and “The Hollow” at Signature and “Candide” at Shakespeare. Posner cast her in plays such as Tom Stoppard’s “Arcadia” and Anne Washburn’s modern version of Euripides’s “Orestes.” Although Seibert rarely played a lead, she worked.

“Before I came to New York, I took my sweet time,” she said, explaining that she always networked backstage. “I asked people in ‘A Christmas Carol’ [at Ford’s Theatre]: ‘Who’s your agent? Would you mind if I passed some stuff along?’ ”

Working with Posner turned out to be key. “I introduced her to her agent, who is my former agent,” Posner said. “I called my former agent and said, ‘You need to know this woman.’ And I believe I said, ‘If she gets the right part, she’ll go all the way.’ ”

Since moving to New York, Seibert has traveled for work to theaters in Vermont and New Jersey. Broadway never beckoned. And then, boom. “My agents called me with the audition for ‘Rocky’ ” — a movie she’d never seen. But in callback after callback, “Rocky’s” directors and songwriters and casting people became more and more impressed. “At every audition she became more credible,” said Timbers, who directed the hit “Here Lies Love” at off-Broadway’s Public Theater and “Bloody Bloody Andrew Jackson” on Broadway. “Every time she came back, the work became stronger. And when we put her together with Andy, you felt that there was really fantastic chemistry.”

For the moment, Seibert is living in the exacting bubble of final rehearsals and early previews — the calming chaos before the inescapable insanity of hype and reviewers and opening night. On that fast-approaching evening, March 13, Debbie Seibert will take her primo seat to the first show on Broadway she will have attended.

“I can’t even imagine what it’s going to be like when I see her,” she said. Knowing though, that her daughter passed every test on the way to becoming Adrian — even getting the thumbs-up from the original Rocky, one of the musical’s producers — helps to settle her nerves.

After all, she said, “I can’t argue with Sylvester Stallone.”