The 32-year-old, who is dating Formula One star Lewis Hamilton, said she is excited about her role in the movie alongside Will Smith.
Speaking to UK newspaper The Sun, she said: 'Auditions are awful. On stage being a pop star I feel at home. Auditions are cold, unpleasant. I have gone through so many and been rejected and sent home in tears.
'But I just finished Men In Black III - a proper Hollywood blockbuster - and it makes all that rejection worthwhile.
'I play Lilly, the girlfriend of the lead character, Will Smith. She is a futuristic vixen of a villain. Oh, and I get to carry a rocket launcher. I am kinda bad-ass in the movie, I'm not going to lie.'
She added: 'Will Smith made me feel like family on set and Emma Thompson is lovely. Tommy Lee Jones was cool too.'