Not so unusually, Jessica Chastain's day began with a good crying jag, admittedly for no real reason. Some security guard had yelled at her for coming in through the wrong entrance. "Even just talking about it now..." she said over tea at Manhattan's Crosby Street Hotel, welling up. "I really get affected by things." Offscreen moments on the set of The Help, her breakout him of last summer, for example, found her apologizing to a chicken, whose character—also a chicken—would be getting her head cut off in the imagined future. ("I'm really sorry," director Tate Taylor overheard her saying. "I just want you to know, I mean you no harm.")

She specializes in tales of self-doubt. "I've spent my life being embarrassed," she says forthrightly. At her first red-carpet appearance at Cannes, last year for Terrence Malick's The Tree of Life, she was plagued with a nagging worry: "Will someone be mean to me?" Talking to fans is great, she says, especially because they're not mean—at least they haven't been yet. Fundamentally, because of all the potential for meanness, and she knows this sounds really odd, "I don't want people to look at me."

"[CHASTAIN'S] VERY SPONTANEOUS AND VERY EMOTIONAL," SAYS DIRECTOR JOHN HILLCOAT.

Indeed, it's an odd, disarming admission coming from a woman who last year hit the screen in four other films as well—The Debt, Coriolanus, Texas Killing Fields, and Take Shelter. "I'm the unknown everyone's already sick of," she insists. But few people actually recognize her from those films, so utterly did she disappear into the roles. This, it turns out, has been the point of acting for her: to leave Jessica Chastain behind. And all the anxiety and vulnerability is part of the mechanism—the fuel for becoming other people.

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This extraordinary gift has made Chastain the name every director and producer is dying to get. "Actors [such as Chastain] who come from the theater, who are highly trained—that can get in the way in films in terms of raw emotion," says director John Hillcoat, who cast her in Lawless, his Prohibition-era gangster movie, out this month. "But she's very spontaneous and very emotional and has a warmth to her." To Hillcoat, she brings to mind Cate Blanchett, another screen wonder who came onto the scene relatively late. (Chastain is 35.) To Al Pacino, who gave her her first break, in 2006, by casting her as Salome to his King Herod in his production of the Oscar Wilde play Salome, she's the next Meryl Streep.

She is, as they say, "on fire," a famously tenuous state that might cause another actress to dive into the first tent-pole superhero movie that comes her way, before it all dies down. Not Chastain. Instead, she turned down Iron Man 3—which would have been her first big payday—to shoot a small project she's passionate about, The Disappearance of Eleanor Rigby, and to prepare for her Broadway debut in the title role of The Heiress, Augustus and Ruth Goetz's 1947 play based on the Henry James novel Washington Square. Veteran Hollywood producer Paula Wagner, for whom The Heiress marks her own Broadway debut, says, "What's really rare here is to have an actress at the peak of her career say, 'Stop, I'm going to do Broadway.' The rigors of that..."

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Chastain initially hesitated to accept the Heiress role, finding every reason to say no. She knew from past stage experience, after she graduated from the Juilliard School, that this would entail a self-imposed seclusion by day. And the shoes she had to fill were large indeed. Olivia de Havilland, who played Catherine Sloper in the 1949 William Wyler movie version of The Heiress, won the Academy Award for her portrayal. The play has been revived on Broadway three times, notably in 1976, with Jane Alexander, and in 1995, with Cherry Jones. How easy it might be to fall on her face. But ultimately Chastain understood that the anxiety meant something important. "Maybe if you're afraid of it, you should look more into it," she thought. And something about Miss Sloper beckoned. "I had a connection to her. She's painfully uncomfortable and I used to be that," says Chastain, who was born and raised in a small town in Northern California, which is as much geographical information as she is willing to give—in order, she says, to protect her siblings, two of whom are still in school.

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This apparent overnight success has paid her dues. After graduating from Juilliard, she did Off Broadway and small television roles playing people who were either "crazy" or "annoying." She landed a couple of plum movie roles, in Malick's The Tree of Life and John Madden's The Debt, but for various reasons, those films languished for years before being released. Finally, in 2010, she got the role that would cause her career to explode: Celia Foote, the naive social outcast and the only white woman in the movie to make friends with her maid. It was, according to The Help's director, Tate Taylor, "the trappiest role in the script." Dozens of actresses wanted the part, a number of name actresses included. "Everybody was reading her and seeing her as this Jessica Rabbit bombshell," he says. But Chastain came to the audition plainly dressed. Octavia Spencer had already been cast as the maid, Minnie, and was reading with her. "Octavia teared up, and I was tearing up," recalls Taylor. "My camera guy was tearing up. I just said, 'That's Celia.'" Released last summer, The Help became the rare drama-comedy that made action-movie money, earning $211 million at the box office.

"I HAVE A FEELING THAT VERY SOON I'M GOING TO FAIL VERY, VERY BIG," SAYS CHASTAIN.

These days Chastain is getting offered every script in Hollywood, and she has been chosen as the face of Manifesto, the new Yves Saint Faurent fragrance. (Up next is Kathryn Bigelow's film about the hunt for Osama bin Faden, Zero Dark Thirty) But, for now, she is immersed in her old friend anxiety. "I have a feeling that very soon I'm going to fail very, very big." Who knows—maybe it will be The Heiress. For the past week she has been going nuts thinking about how to crack Catherine Sloper's character. She claims she's coming up empty-handed. "I haven't been this unsure in a long time. I feel really, really terrified." All the signs, in other words, point to success.

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FROM THE ARCHIVE

For these related stories, visit VF.COM/ARGHIYE

• Making The Help (Krista Smith, July 2011)

• Jessica Chastain's Vanities debut (Krista Smith, December 2009)

• Behind the scenes at this year's Hollywood Issue cover shoot (Krista Smith, March 2012)