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At another point in our conversation, Atwell uses those exact same words about Whishaw, whom she judges the best actor of his generation. But of all her leading men to date, there can surely be no smoother operator than Colin Farrell, with whom she appeared in Woody Allen’s Cassandra’s Dream. Dame Eileen Atkins famously disclosed that the actor, 28 at the time, tried his considerable best to seduce her when they made a film together just before her 70th birthday. Surely, I suggest to Atwell, he must have made a move on her too? She laughs out loud and then prevaricates. “The thing is, he’s a lovely, lovely man and so can get away with absolute murder. The charm. The charisma. He has this most extraordinary ability to make any woman – or man – to whom he’s introduced feel like the single most important person in the room.”

But did he make that move? “At the time, I was going out with someone who’d worked with him years before. When I mentioned this in conversation the first time he, I and Ewan [McGregor, their fellow co-star] had dinner, Colin said, ‘I love that guy! He’s great! And it’s so cool you’re with him ’cos now I won’t have to worry about trying to f*** you!’, to which the only reply I could think of was, ‘Oh. OK. Right.’” Atwell then offers two examples of what she describes as Farrell’s characteristic thoughtfulness. After that meal, for instance, he insisted on delivering her by cab to her boyfriend’s door. “Then, when the film played at the Toronto Film Festival and I took along my aunt, he took care of her all evening.” And she would be what age? “In her sixties. Why?” She pauses, reflects and then hoots. “Oh my God!”

When called on to play a sexual initiator herself, Atwell took inspiration from another source entirely. The Duchess contains a scene in which her character seeks to enlighten Knightley’s heroine, trapped in a joyless marriage, about the potential for pleasure within sexual union. “We set out beforehand what we were comfortable with, Keira saying, ‘Whatever, it’ll end up on YouTube as my lesbian sex romp.’ But women were so much more affectionate with each other back then because they didn’t get that from their partners… It reminded me of Billie Piper, with whom I’ve worked. She’s very tactile and has this natural sexual energy that’s overpowering. Around her, I found I’d feel incredibly turned on. Yet there’s also an innocence, which is what this particular scene was about. So I based myself on Billie. I don’t know if she’ll appreciate that or not.”

No, Atwell says when I ask, she herself doesn’t have a partner right now. “I’m single. My only significant other is my dog.” Who, it turns out, is about to be left in the care of friends as she heads off to Namibia to film scenes for a remake of the Sixties sci-fi drama series The Prisoner, in which she will appear alongside Sir Ian McKellen and Jim Caviezel. Experienced actors must dream of being cast in such roles as she now habitually is, let alone those still waiting for their first break. “I know, I know. There’s a fear that as you accumulate more of a profile, you expose yourself to the possibility of negative reactions. It’s just the politics of the playground though, isn’t it, and I’ve weathered those. And anyway, someone new will soon be coming up behind me. Some other girl fresh out of drama school is already lining up to get all the parts.”

Brideshead Revisited is released on October 3

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