The PIRATES OF THE CARIBBEAN star was convinced she wouldn't impress in a lap dancing scene in the film after disastrous lessons with a real club girl.
And then her mum, a former go-go dancer, came to the rescue.
Knightley says, "My mum used to be a go-go dancer. She got fired, apparently.
"She kept saying, 'Oh, be sexy, be sexy'. I'm thinking, 'This is so weird, my mum is telling me how to lap dance'."