"Getting in and out of cars with the wigs on was difficult, we had to slot our heads through the door sideways," says Atwell. "Keira (Knightley] had to wear this huge wig that was so heavy that they made her a special board that she could lean her shoulders and head against in between takes without toppling over. Going to the toilet was like a game of unwrapping pass-the-parcel. It was absolutely extraordinary to think that's how they dressed every single day."
Based on Amanda Foreman's biography, The Duchess chronicles the unhappy love triangle that Georgiana, Duchess of Devonshire, shared with her husband and his mistress. Knightley stars as the Princess Diana of her day, while Ralph Fiennes plays her unfaithful spouse. Yet it's Bess, the third corner of the triangle, who is the most fascinating, managing to captivate the husband but also make a lifelong friend out of Georgiana.
Brideshead Revisited has much the same air of clenched reserve, with Atwell cast as the flirty, bobbed Julia Flyte, with Emma Thompson and Michael Gambon as the estranged Flyte parents, Ben Whishaw as Julia's doomed twin and Matthew Goode as the Marchmains' enraptured narrator, Charles Ryder.
When ITV screened its lengthy adaptation of Waugh's nostalgic 1945 novel, Atwell hadn't even been born. But that's not why she hasn't seen the 1981 series that made stars of Jeremy Irons, Anthony Andrews, and Diana Quick. "We wanted to make a film adaptation of the book as opposed to a remake of the television series, and I thought the only hope I have is that if I don't watch the series so that I don't end up trying to imitate the brilliant Diana Quick," says Atwell.
Running counter to the arching melancholy of the book, the set of this Brideshead appears to have been a light-hearted one propelled by Emma Thompson, an enjoyably bossy co-star who took the young cast to lunch, dinner and parties, as well as a traditional Latin mass – although they dodged communion in favour of a Sunday roast back at Thompson's house.
"She's brilliant and funny and very much into empowering younger people," says Atwell, who discovered this first-hand when it was suggested that she lose a few pounds before donning the bias cut dresses of the period.
"Emma invited me around for a meal and was very cross when I didn't finish my dinner," she recalls. "When she found out the reason, she was furious with the studio and told them I was just fine as I was. After that, they left me alone."
Over the course of her last three films, Atwell has become adept at translating British customs onscreen. Last year she had to fill Woody Allen in on some details when she worked with him on Cassandra's Dream, opposite Ewan McGregor and Colin Farrell.
"You've seen the films; do you think Woody understands England?" Atwell laughs. "Casting a Scot and an Irishman as Cockneys gives you an understanding of how open he is to a new interpretation of the English class system."
Atwell had just graduated from London's Guildhall School of Drama when she flew to New York to meet Allen and audition as a stage actress who captivates Ewan McGregor's wide boy.
"You just go into this room and you're given about three lines of dialogue and all you have to do is look at the person that's reading with you, do the dialogue, and then leave. You have no sense of who the character is, what the film is called or who the other actors are, or what the situation is. I left thinking, "that was hideous, I'm never going to work again".
"It's very intimidating because he doesn't talk. There's no rehearsal, there's no read-through, there's no socialising. You do it in two or three takes, and you're home by 4pm.
Atwell's career has evolved at a rapid pace as well. She has worked with the Royal Shakespeare Company and gave an eye-catching performance as the wayward daughter of a Tory MP in 2005's Line Of Beauty. With a string of films and a run in the lead role of George Bernard Shaw's Major Barbara at the National Theatre under her belt this year, Atwell cautiously acknowledges that she's achieved a lot in a short space of time, yet has no plans to move to Los Angeles any time soon. "I have a team of people who know what I want to do in terms of scripts," she says serenely. "I love England and that's where I'd want to centre myself."
Her next project is "something I don't wear a corset in" – a remake of TV's innovatively perplexing series The Prisoner, with Ian McKellen and Jim Caviezel as Number 1 and 6, and Atwell as Caviezel's ambiguous love interest. Is she looking forward to being pursued around Portmerion by a giant yogaball?
"Well, actually I'm going out next week to start filming in Namibia, not Wales," she grins apologetically. "And we've been given a list of activities like a camel safari, skydiving and dune surfing, which is great because usually they worry about you breaking a leg and holding up filming. So as well as being chased by the balloon, I'm hoping I'll get to take a hot-air balloon across the dunes"
• The Duchess is released on 5 September. Brideshead Revisited follows on 3 October.