Kiera Knightley, star of Pirates Of The Caribbean and Atonement, explains why it was terrifying to play Georgiana, Duchess of Devonshire in The Duchess.

Keira Knightley clearly learned one thing from portraying infamous 18th century socialite Georgiana, Duchess of Devonshire, in her latest film The Duchess - that she needs to take full advantage of her modern right to breathe.

“The costumes were completely fantastic because Georgiana was a very famous fashion icon. But they weren’t particularly comfortable,” she says frankly.

Considering she plays an aristocrat whose glamorous, yet tragic, life has led her to be compared with the late Princess of Wales, this was not such a bad thing.

Author Amanda Foreman is largely responsible for bringing the duchess' life back into the spotlight, thanks to a biography she wrote in 1998.

Details about her husband’s infidelity, her trend-setting outfits, affair with the future prime minister Lord Grey and eventual menage-a-trois with her husband and former best friend Lady Bess Foster, proved sensational reading and the book was an instant hit.

The film version, to be released on September 5, stars Keira Knightley along with Ralph Fiennes as the Duke, Dominic Cooper (The History Boys and Mamma Mia) as her lover Lord Grey and Hayley Atwell (The Line Of Beauty) as Bess.

Although the film's producers are hoping that linking Diana, Princess of Wales with her great-great-great-great-aunt Georgiana will help attract large audiences, Keira says she didn’t play on the connection.

“I was 11 when Diana died. I knew when I was going into it that she was a distant relation but that's as far as my knowledge went. I was making a film about Georgiana Duchess of Devonshire and I think she's an interesting enough person to warrant a film about her, without comparison."

“I first heard about the film when Saul [the director] sent me a letter with three really big ostrich feathers tied with a big bow,” says Keira.

“They were very pretty and I thought I have to work with a man who does that.”

The feathers were a nod to one of Georgiana’s more flamboyant and original head-dresses which helped turn her into one of the most talked about women of her time.

“When I read the script the first thing that struck me was how incredibly lonely this woman was,” says Keira.

“She was constantly surrounded by so many people and yet entirely alone. I think she just tried to grab on to any kind of love and attention that she could possibly get.”

Shot over nine weeks in the autumn of 2007, the film is set in three principle locations, the Devonshire's London home, Devonshire House; their country estate, Chatsworth and Bath. Several locations were used to recreate their lavish homes including Kedleston Hall in Derbyshire, Holkham Hall in Norfolk, Osterley Park, Somerset House in London and also Chatsworth, the ancestral home of the Devonshires, where the current Duke and Duchess still live.

“Being in those enormous houses, helped with the feelings of isolation that I thought Georgina was going through. The sheer scale and beauty of them is quite astounding,” Keira says.