The 23-year-old actress trained with a couple of members of the United States Armed Forces in preparation for her role as an army guard at Guantanamo Bay who befriends a prisoner in 'Camp X-Ray' and admits she found the experience extremely tough.
The brunette beauty told E! News at a screening for the film at the Sundance Film Festival in Park City, Utah: "I spent a few days with a couple of Marines that really put me through the ringer. It was awesome. They're great guys.
"We wanted to represent them in the best possible way we can and if I was remotely unauthentic, I would die. I would hate that. We did everything we could to keep it real."
She added: "I had to move and respond like a soldier that was not given any room to breath. She's not using all the things she's been trained to do. She's walking around in circles. I needed to look like I learned those things but couldn't really display any of it."
Kristen frequently visited a driving range to help her unwind after filming intense scenes.
Her co-star Lane Garrison, 33, said: "We built a driving range in the prison and I brought like 400 golf balls. We would go out there and hit about 100 golf balls. She has an amazing swing."