MONTHS of being sewn in and out of 18th-century corsets have clearly taken their toll on Keira Knightley.

The 23-year-old arrives to meet eager journalists in what looks like a full-length frilly black nightie.

Keira 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. “In a corset you can’t catch your breath. It means your emotions are heightened because you can’t calm down.”

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 stars Knightley along with Ralph Fiennes as the Duke, Dominic Cooper as her lover Lord Grey and Hayley Atwell 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.”

The beautiful and charismatic Georgiana was married at 17 to the Duke of Devonshire – a man more interested in sleeping with the maids and playing with his dogs than his new wife – and for many years was unable to bear him an heir.

Her husband ignored her as a result and her dreams of a fairytale life gradually died. But despite her personal problems, the young duchess won the heart of English society, and only the Duke seemed immune to her appeal.

“I first heard about the film when 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.”

Naturally enthusiastic and intelligent, Georgiana became involved in politics and eventually fell in love with the future Prime Minister, Lord Grey. But despite having to accept her husband’s mistress in a humiliating and extremely public menage-a-trois, her love for Grey was forbidden by the Duke.

“The whole journey for her is one from idealism to reality – she gets broken down. She couldn’t have lived with herself if she’d gone off with Grey and given up her kids. I don’t know if it has a happy ending but I think that she survives and that can only be positive.”

Although the 18th-century paparazzi at that time constituted a bunch of talented cartoonists, as one of the most powerful, beautiful and controversial ladies in the land, the Duchess’ life was still the subject of endless gossip column inches and public speculation.

“The current Duchess of Devonshire showed me some of Georgiana’s things. She was a huge gambling addict, so it was amazing to see her books of debts written in her own hand,” Keira says.

As well as her love of cards and extravagant shopping, the Duchess liked to socialise with society’s opinion makers.

The public adored her unique combination of intelligence and wilful self-destruction.

Keira admits that playing the Duchess wasn’t easy.

“It was completely terrifying. It’s very rare to get a role like that, so when it comes along and you get the opportunity to play it, it’s phenomenal and terrifying.”

Although she had a lot on her plate, Keira did let her hair down occasionally during filming – but not literally, as the authentic 18th-century headdresses needed to be painstakingly unpinned.

“During the sex scene with Grey I was luckily fully clothed but he got to wear a skin coloured nappy. On set, you’re supposed to be supportive of each other but when he came out I completely lost it!”

Having played such a wonderful role, does she hope more similar projects will come along?

“Recently I do seem to be getting sent books and ideas for stronger female characters, which is completely fantastic. I think if people go and see films about strong women, there’ll be more made and if they don’t, there won’t. It’s up to the public.”

The Duchess is out on Monday