NEW YORK -- As the stage lights dim at the Walter Kerr Theatre, signaling an act break for "The Heiress," actress Jessica Chastain gets up off the floor and exits stage left. She sniffles back the tears she effortlessly manifested for the previous scene, preparing for the next act. Her character, Catherine, is frail, emotional, precious, and at the end of this act, burdened by the unloving eye of her father and twisted-up passion for a would-be beau. One can't help but think, "Maya would never be in this position."

Maya is Chastain's character in Kathryn Bigelow's "Zero Dark Thirty," a dense and principled account of the hunt for Osama bin Laden. She's driven, single-minded, seemingly without emotion, save for the tears she can finally shed when her mission is over. It's a fascinating foil to Catherine, who spends the entirety of "The Heiress" moving to a place of rigid, emotionless resolve. And so while on the stage Chastain is performing a fragile character's journey of clenching up, strengthening and hardening, on the screen she's performing a hardened character's journey of releasing, letting go and softening.

"I think the reason for that is mostly I’m just interested in characters that have a huge arc," Chastain says a few days later over lunch at the Mercer Hotel. "But it’s interesting also that Catherine is a woman that defines herself completely by men, because she sees herself as her father sees her in the beginning. And then immediately transfers it to her fiancée. Whereas Maya…for her it’s a badge of honor to stand alone. She sees it as this great accomplishment that she’s completely focused on one thing."

Performing a character like Maya -- who is based on a real-life CIA agent whose identity has been kept a secret as she's still in the field -- is much more difficult than performing, say, the colorful Celia Foote in last year's "The Help" (which netted her an Oscar nomination), Chastain says. But the reward is in the digging, and she confesses a passion for research in that regard.

"I'm playing someone who is trained to be unemotional and analytically precise," she says. "So it means I can’t follow my instincts. I’ve been trained my whole life to be emotional, be vulnerable, to allow whatever hits me to really affect me, but in really intense interrogation scenes when I’m watching Jason Clarke [waterboarding a subject], my impulse is to really be in it, vulnerable. But I have to then take that and say, 'Well, how could I take what I'm feeling and not lie about it?' I have to take it and then go, 'My training doesn’t let me show exactly how upsetting this is to watch.' It has to be detailed. I can’t do the same scene 10 different ways."

It's a unique character also for its overall lack of backstory or context outside of the events portrayed in the film. Further complicating matters -- or perhaps liberating them -- is that producer and screenwriter Mark Boal, who researched and interviewed the story extensively, has to protect his sources, even to the actors who are portraying them. If Chastain were playing, say, Marilyn Monroe (who she studied at length for her role in "The Help"), there are hours and hours of footage at her disposal. But the caveat there is in mimicry. "How do I create something," she ponders of such studies.

"I tried to give Jessica and the other actors an idea of the people involved through the characters they play in the script," Boal said in an email. "Whenever Jessica had questions about the research behind her character, I tried to give her a general idea of what life was like for people in the CIA, as best as I understood it. But beyond that, I couldn't draw a dotted line between anyone in the script and anyone in real life because these are civil servants, not public figures."

So with no access to "Maya" and only a certain amount of context from the producer and director of the film, the danger of mimicry may not have been there, but the challenge to build character was as apparent as ever. Nevertheless, Chastain says she relished the process, however unique, because it played into her own habits as an actress.

"Something that I do share with Maya is I really like research and I really like solving puzzles," she says. "I would read the script and I found a lot of clues. Like I could take any scene and take one line and go, 'Well, this line means this about her.' I have a process: whenever I do a movie, I make a list of everything all the other characters say about my character and everything my character says about herself, and just looking at that list tells you so much about the person you're playing and how they're different from you. 'Washington says she's a killer.' That's something someone says about Maya. 'A lot of my friends have died trying to do this; I believe I was chosen to finish the job.' Someone who believes that they were chosen to finish the job -- that says a lot about her."

In addition, the practical details that pepper the script became in-roads into the character in many ways. Chastain recalls one as an example, Maya's favorite candy -- Twizzlers -- and how something that small can provide a framework for an inner context. "All these things that call back to this life that Maya then becomes a stranger to, having that there means while we're in the moment, we can create scenes around it," she says. "Like that candy scene. There’s no lines or anything, but for me, when we’re filming that, coming in from the market, wearing that black robe, sitting down on the couch, drinking the Diet Coke and eating those Twizzlers, it's like this contradiction, of the old world and the new world for her."

She pauses and needlessly apologizes. "Sorry. I talk a lot about acting because I really love it. So if I start talking too long you can just cut me off."

But that spotlight and, indeed, reliance on the character's actions above where she comes from or who she is in a contextual sense is in some ways the essence of cinema. "Shakespeare's even like that," Chastain's co-star Jason Clarke said in a separate interview. "The thought is on the line. There's not subtext; it's just there in the line. What you say is what you mean and what you mean is what you do…You are the sum of all your actions."

Which brings us to that single tear by film's end, and a closing line Chastain has frequently noted as resonant for both the character and the greater themes of the film.

"It's not just like a propaganda, 'Go America,' fist-pumping thing," she says. "It ends with a question. 'Where do you want to go?' And she doesn't know. There's an emptiness. And it's more than just her because it represents us as the audience. Where do we go now that we've killed bin Laden?"

Maybe we go into a certain state of reflection. Much has been made of the film's depiction of torture, leading it to become a political football amongst agenda-driven outlets looking to kick it around. But the depth of its matter-of-fact handling in the script is what resonated for Chastain.