The 13 year old was cast as assassin Hit Girl, a character who uses bad language throughout the movie and is seen savagely killing numerous villains.
To prepare for the physically intense role, Moretz learned martial arts and spent two months in a challenging training program, prompting co-star Aaron Johnson to dub her a "mini-Schwarzenegger".
She says, "Two months before the movie started I started training to be Hit Girl. I knew it was going to be physical, but I didn't know it was going to be that physical. First I did basic training - how to take apart a gun and how to put it back together. I then did more technical training, like the bow staff and butterfly knife. They made me do 1,000 crunches a night and 50 pull-ups. I loved learning the martial arts moves. Learning to flip a butterfly knife was fun and challenging too."
But despite her role, Moretz is adamant she hates bloodthirsty films: "I would never want to see that kind of violence. I'm a chicken. I'm scared of spiders. I like romantic comedies. I'm totally a chick-flick girl."