The 2003 movie was a collaborative film starring Knightley, Liam Neeson, Bill Nighy and Emma Thompson, and is regularly shown on U.K. TV in the lead-up to Christmas Day.
And Knightley loves having a film which has become a must-see for families at this time of year.
She tells the Huffington Post, "It's amazing that it turned that kind of Christmas movie, isn't it? I don't think anyone realised it was going to do that.
"It's an extraordinary thing when people come up and go, 'Oh, you know, that's the thing I watch every Christmas'. It's a lovely thing. That's why you make them."