Knightley praised the sincerity of the SIN CITY star, who appears alongside her in forth-coming movie Domino.
The 20-year-old says, "Working with Mickey was amazing. He's completely honest with you - there's no facade.
"The thing I love and admire most is that he makes absolutely no pretence at perfection.
"He can't help but be himself. There are very few people in this industry who can do that."