In an interview in that tower of journalism, USA Today:

She and [Ralph] Fiennes had never met before the film and in the movie have no loving, or even coolly affectionate, scenes together. His duke lavishes warmth on his dogs but dismisses her. Her duchess shacks up with a younger and far more attractive lover. He threatens to take away her kids. She refuses to save the marriage. In real life, both say they got along just fine. “To play a married couple that have no familiarity with each other was an interesting test,” Knightley says.

So, how did she keep the tension and rancor alive even as she got to know Fiennes better?

“I don’t know. It’s a very American question, how. It’s (expletive) acting!” retorts Knightley.

I love Keira Knightley.

(I want to see this movieeeeeee. But nobody says when it opens wiiiiiiide. Aaaaauuughghgh.)