Keira Knightley hates being told to "relax" when she's acting.

Keira Knightley

Keira Knightley

The 33-year-old actress says people often try to remind her that "acting is relaxation", but she finds it difficult to switch off when she's on set, and only discovered it was acceptable to make mistakes when she watched Dame Judi Dench filming in 2005 film 'Pride and Prejudice'.

When asked what the best advice she's received is, she said: "I've been told so many times 'Acting is relaxation, acting is relaxation,' but you f***ing try it! And the more you try to relax, the more tense you feel. But on 'Pride and Prejudice', [Dame] Judi Dench forgot her lines in our scene and I suddenly realised, 'Oh, actually, mistakes are fine.'"

The 'Colette' star received criticism for her role in the flick when she was first cast, but says she managed to brush it off thanks to her "f**k you attitude".

She added: "I think I've always had a 'F**k you' attitude, and that's helpful sometimes. With 'Pride and Prejudice', everyone was like, 'She can't act and that's going to be dreadful.' So I thought, 'Well f**k you!'"

And Keira sometimes finds it hard to leave her characters on set, as she says the 2012 movie 'Anna Karenina' saw her bring the titular character home, much to the annoyance of her husband James Righton.

Asked by Notebook magazine if she ever finds it hard to switch off, she said: "Yes, in 'Anna Karenina'. It was technical to film, so it was the only time I've ever - not intentionally, but totally - taken the character home. And my then boyfriend, now husband - f***ing miracle that we stayed together after that - was like, 'I really don't want to live with Anna Karenina ever again.' I terrified the cast too. It was a bit much!"