Even though he and Knightley are hardly buddies, they did click during the shoot. "She has a great maturity and an openness and a natural spontaneity, a vivacity. That's a crucial thing you can't act. She has it," he says.
She doesn't moan about the paparazzi attention she gets at home in England and dismisses her peers who act surly and dismissive in public. "They hate their jobs," she says with a laugh.
Yet she draws a firm line between business and her own real life. To wit, she has never talked about her boyfriend, actor Rupert Friend, and won't even confirm she has one.
Knightley buries herself in books and has a penchant for the bleak. Recent reads include Bernhard Schlink's "beautiful" "The Reader" and Richard Yates' "Revolutionary Road." "Two really depressing books but really good," she says. "Not exactly a barrel of laughs."
This time last year, she was devouring Gitta Sereny's "Into That Darkness: From Mercy Killing to Mass Murder," an account of Treblinka's commandant Franz Stangl. Now, she has moved on to the works of novelist and war correspondent Martha Gellhorn.
"Not exactly happy, but interesting. Really great writing creates such amazing images," Knightley says. "It's a compilation of investigative journalism. She has just gone into Dachau. It's awful, but the way she writes is wonderful. I just started it. I'd never read any Martha Gellhorn before, and she's a wonderful writer."
She'll have plenty of reading time after she's done promoting "The Duchess." Knightley has nothing lined up for the rest of '08.
"I haven't quite found what I'm looking for yet," she says, adding that her selection process is more instinctive than calculated. "What I'm not capable of doing is, 'For my career, I should be doing this right now.' I can't do that because I can't try and be interested in something that I'm not."
Nor can she fake enthusiasm for a film she despises when it's time to go out there and sell it.
"It has to be about something that I do actually want to talk about. It can be really embarrassing and depressing and very cynical if you sit there and go, 'Well, I got a really big paycheck.' "
Knightley isn't all doom and gloom. In fact, when it comes to films, she says the oddball and offbeat move her. "There's an amazing film called "Couscous" (La Graine et le mulet) — so beautiful, about a guy who's trying to open a couscous restaurant," says Knightley. "I sat in the cinema crying and crying and crying. I want to phone the guy up and say thank you.
"Film can be so magic."