December 21, 2012 at 05:00 AM EST

I knew I would be playing a real woman who’s an active member of the CIA and that I would not be able to meet her. So I relied very heavily on my incredible director, Kathryn Bigelow, and screenwriter, Mark Boal. Mark began his career as an investigative journalist. I spent three months before shooting asking him questions about my character and the CIA. I also read many books. Anything I couldn’t find from the research, I had to use my imagination to fill in the character.

This was the most difficult text I have ever worked with; I studied Shakespeare at Juilliard for four years, but this seemed harder to me. To play this CIA agent, Maya, I had to give these lengthy, technical speeches and understand every single word. And at the same time I had to play the character’s arc. I wanted to show this woman descending into the rabbit hole: this loss of self, becoming a servant to her job and to the work.

It was all supertough to shoot, but the interrogation scenes were the toughest, because I don’t like confrontation and we were filming in an active Jordanian prison on the outskirts of Amman. Imagine going to work every day and there are barricades and guards. You can’t take a cell phone, you have to be searched before you go in, and there were certain times of the day I was not allowed to be outside.

Doing those scenes was rough, and I couldn’t purge my feelings through my character — I couldn’t be in the scene and have a good cry because Maya couldn’t do that. I’m playing a character who’s trained to be unemotional and analytical, but I’ve been trained to let the walls down and open my heart and be vulnerable.

I loved this character so much, so I wanted other people to connect to her. Maya is a tough woman because she doesn’t take no for an answer. No matter what gets in her way, she won’t back down. She’s capable and strong, and she stands on her own. It’s exciting to see that representation of women in a film.

Because she’s an active member of the CIA, she can’t take credit for this amazing thing she did: being instrumental in finding Osama bin Laden. So for me, making this movie is giving her credit. It’s thanking her for her service. It’s thanking everyone involved. There are hundreds of people?thousands of people?who worked on this mission for a decade, over three continents, to find this man. And Zero Dark Thirty is our way of thanking those unsung heroes. —Jessica Chastain

Zero Dark Thirty is in theaters now.

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