Pettyfer was reluctant to accept the part of an alien in the upcoming sci-fi film, fearing he wouldn't be able to carry the big-budget blockbuster, which was produced by Steven Spielberg.
And the 20 year old admits he was so worried, he walked out of his audition before he'd begun reciting his lines - and was shocked when he later landed the role.
He tells Britain's Empire magazine, "I went for an audition and before I even started saying the lines, I looked at them, stood up and said, 'I'm sorry I'm not right for this.' And walked out! Deborah Aquilah, who's the casting director, ran after me, and said, 'Alex, Alex... You've got to come back!' I said, 'I can't I don't want to muck up your movie.'
"They called me two weeks later and I still didn't want to come in, but they hadn't found anyone. I read for D.J. (Caruso, director) and he seemed to like what I did and screentested me. I got an email from my agent that night that said, 'We're waiting to see what Steven says.' I was like, 'Who's Steven?' 'Steven Spielberg...' 'Oh, OK...' And he gave me the thumbs up, which is a big honour."