The actor plays Number Four/John Smith in the D.J. Caruso-directed film, but told Sugar that he initially thought he could not win the role.
Pettyfer said: "I actually walked out of my first audition because I didn't think I could do it. Working with people I idolise puts me on edge.
"But I went back and hopefully I didn't embarrass myself... Well, I must have done something right."
Of the part, he added: "I really enjoyed being a different kind of hero. It's James Dean, mixed with Bourne. He can do all this amazing stuff, but doesn't want to. He's a reluctant warrior - he just wants to be normal."