"I didn't really know of Lydia before I cast her. I think I'd seen her in one small thing, as an Eastern European," said 's director, Jeremy Herrin, an Olivier-nominated British theatre veteran. "But I was quickly struck by her. She had this mix of cold intellectuality and a strong emotional sense you don't often find in the same actor."

Leonard's success underscores theatre's ability to mint new stars from seemingly out of nowhere, even as her decidedly modern style of gregariousness reminds us that actors on stage can be very different off stage.

As she walks through midtown Manhattan's theatre district on a recent weekday, the diminutive Leonard proves a spark-plug presence, as taken with the swirl around her as many of the tourists who unknowingly passed her by. She is on her way to work after an actor's happy-hour dinner (at Sardi's, because it seems the kind of prototypical Broadway-ish place she'd read about in England).

Leonard speaks quickly and conversationally, a contrast to the Machiavellian pronouncements of her character. She mentions the friends razzing her on Facebook for all the morally slippery adjectives critics have bestowed on her character. She cites the problems a carb-based meal can present to a corset-based costume, and ponders whether her chosen castmate gift (bandannas) was on par with other gifts (customised T-shirts, roses).

She spots a billboard. "Oh, there I am," she says, with a mixture of satisfaction and surprise as she gestures to a image. On it, a frocked, steely-eyed Boleyn dominates the foreground while Nathaniel Parker's Henry and Ben Miles' lead character, Thomas Cromwell, hover behind. "Well, that's a little embarrassing," she says. "Why am I taking up so much more space than Ben and Nat?"

This is a comparatively easy day for the actress - only a single two-hour, 45-minute show, compared with the 51/2-hour, two-part marathons of other days.

Indeed, has become a phenomenon as multifarious as Henry VIII's matrimonial activities. Hilary Mantel's 2009 historical novel of the same name - about how the humbly rooted Cromwell plotted his way to behind-the-throne power, with Boleyn as ally and antagonist - won the Man Booker Prize, spawned a sequel (), a planned third book this year ( ), a BBC miniseries based on both books, and a two-part Royal Shakespeare Company staging.

After a pair of go-rounds in England, the Royal Shakespeare Company production, with most of the cast intact, opened at Broadway's Winter Garden Theatre in April to sparkling reviews. On Wednesdays, Saturdays and Sundays, the two parts are performed consecutively, with just several hours of break in between. (Thursday is reserved just for the first part and Friday the second; both performances are in the evening.) But what could have been a formal stunt turns into more than the sum of its parts, a live-theatre spin on binge-watching. "We're part of an offering that pushes back at how tiny our goblets of culture are becoming," director Herrin says.

Because Boleyn is as much a factor offstage - players frequently worry about what she will or won't do - as on it, Leonard has some breaks between showstoppers. Still, she commands the room much of the time she's in it. Leonard's character makes dramatic pronouncements such as "those who are made can be unmade" yet tempers her brassiness with vulnerability.

"Anne is strong and bold and often vicious, but I wanted to investigate her appeal to Henry - not to fall into the trap of making her likeable, because that's irrelevant and not interesting - but to keep it fun and see how clever and bright she is," she says.

Leonard's career began in drama school, and she has often starred in period pieces in England, largely on the stage (for example, a Royal Shakespeare Company production of ), although she did have a small role in the Bill Condon-directed WikiLeaks movie .

Herrin believes Leonard's success comes from her willingness to hold her own in a male-dominated world, both in the show and beyond. "There's something about her that's very comfortable with men - something dynamic and tough and a little tomboyish and competitive," he says. "She fights for things; there's nothing demure or coquettish or backfoot about her."

Co-star Miles chuckles when asked about her demeanour. "She says exactly what she thinks. Always in the nicest way. But she is always ready with a comeback." (Miles, who spends nearly the entire 51/2 hours on stage, is best known for the British sitcom .)

Shortly before curtain, Leonard moves quickly around her dressing room - it's a small space, pinned with theatre quotes and cards from friends - as a crew member helps her into a corset.

"The play makes a lot of references to Anne as flat-chested, but this thing has a tendency to push them up," she says, laughing and motioning to herself, then heading down to the wardrobe area. Nearly a dozen dresses hang there. She has a costume change after almost every scene, and many of the outfits are so heavy, they caused back pain earlier in the production.

The pièce de résistance is the crown, placed with great ceremony on her head during the coronation scene.

"It's really not quite big enough," she says as she puts it on, playfully sliding the bejewelled accessory one way in a hip-hop pose and another to look like a court jester.

"But I guess they really liked it. I'm always worried it will fall off while I'm walking. I don't know what I'd do if that happened."

She pauses. "I am Anne. I guess I probably would make someone else pick it up."