The 23-year-old actress said she was sceptical about people's response but was also excited to play a "larger than herself" character, reported Contactmusic.
"I was so nervous when I first started working and preparing because I didn't think that people would believe me or think that I looked tough in a uniform," Stewart said.
"I was excited to play someone, who was standing up to something much larger than herself," she added.
The film follows a young woman who joins the military and ends up as a new guard at Guantanamo Bay prison.