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."

Kristen Stewart takes on role as prison guard at Guantanamo Bay in "Camp X-Ray"