SHE IS one of the most radiant beauties of her day. Wherever she goes crowds gather, temperatures rise and, occasionally, unseemly scuffles erupt. Reporters chronicle her every move; snatched images of her fill the popular press. As a fashion icon, the public can't get enough of her. But adoration has a dark side: being assailed by cries of "whore" by those resentful of her fame is a daily hazard. Such was life for Lady Georgiana Spencer, the 17-year-old who, upon her marriage in 1774, became the Duchess of Devonshire, mistress of Chatsworth House, vote-winning ally of the Whig party and one of the most influential women in Regency society. What, one wonders, drew Keira Knightley to play her in the lead role of new period drama The Duchess?

"I thought it was fascinating because as far as celebrity culture went, it was actually a modern phenomenon," the actress replies. "There would literally be riots when she turned up at political rallies, people following her everywhere trying to draw really quickly what she was wearing, and obviously all the Whig papers were hugely supportive and the other papers were calling her a whore."

Even 200 years ago, it seems, the public were transfixed - bedazzled - by fame. Already, the lumpy proletariat were foisting their aspirations on the sleek and chic.

"Absolutely" beams the 1000-watt smile that last year helped the 23-year-old star of Pirates Of The Caribbean and Atonement to earn $32 million, making her the second-most financially successful actress in the world after Cameron Diaz (at least according to Forbes Magazine). Put this figure to her and she splutters: "I wish. $32m? That's a big load of shite."

She continues: "And the whole concept of a fashion icon, I didn't realise that that was back then too, this way that we have of picking women apart and analysing everything about them."

Poring over paparazzi pictures, circling their wonky toes, giving them marks out of 10 for their elbow skin, agonising over their weight loss crisis?

"Yeah. Which men have never had. I find it quite interesting that we still do exactly the same thing to women as they were doing back in 1790."

So society hasn't moved that far from a Regency-era patriarchy?

"Well," Knightley says with a don't-be-silly arch of eyebrow, "Obviously we've moved on, but the fact that you can find any similarity is quite weird."

That, however, is what happens when society plonks aesthetically pleasing people on big, wobbly pedestals. We may have come a long way from late-18th century England, where society was hobbled by class division, disenfranchised womanhood was trampled down and even the most celebrated ladies du jour were pursued by exploitative men. But in some ways, we haven't come far at all. Keira Knightley knows all about that.

We meet in a London private members' club. Later, as she leaves, she will be "papped" trying to enter a car and in the next days' papers she will appear hunched and hidden behind her curtain of brown hair in the middle of a burly throng. But for now she looks relaxed, sitting cross-legged in a big chair wearing checked shirt, black jeans and fancy cowboy boots. Work-wise, she seems to have no pressing commitments. Yes, she has auditioned for the part of Eliza Doolittle in Cameron Mackintosh's remake of My Fair Lady, but no, she doesn't know if she's got it. Yes, next year she is "nominally" playing Cordelia in a big screen adaptation of King Lear with Anthony Hopkins, "but it's not confirmed".

She often seems a little distant and mistrustful in interviews so I've brought her a present: a copy of Under The Skin, a gripping novel by Highlands-based author Michel Faber. The lead character is a woman who cruises the A9 picking up male hitchhikers.

Knightley is very chuffed with it. A voracious reader despite her dyslexia, she loved Faber's The Crimson Petal And The White. "He just completely gets inside the heads of characters. And it's really grimy," she says approvingly. I tell her Jonathan Glazer (Sexy Beast) has been trying to film Under The Skin for a while. I also mention that, as a Scottish parent of England-born children, I'm always trying to inculcate Scottishness, and Scottish art, in my offspring.

"Oh, my mum does that too," the London-born actress says of Sharman Macdonald, her Scottish playwright mother (her father is theatre actor Will Knightley). Aged 14 Knightley - who began acting at the age of seven - performed in a production of Macdonald's After Juliet. But their first full mother-daughter collaboration came this summer with The Edge Of Love, Macdonald's dramatisation of the relationships between Welsh poet Dylan Thomas (Matthew Rhys), his wife Caitlin MacNamara (Sienna Miller), his childhood sweetheart Vera Phillips (Knightley) and Vera's soldier husband William Killick (Cillian Murphy). The film, a melodramatic chamber piece about indolent bohemians in wartime, divided critics. Some objected to the factual liberties seemingly taken with rumoured aspects of the relationships; some didn't like the stagily poetic script. But Knightley is defiant and protective.

Macdonald - her daughter calls her Shar - "writes amazing dialogue, and you very rarely get really good dialogue in films. The rhythms, and everything were beautiful. She was so f***ing clever," Knightley adds. For all her porcelain poise, Knightley swears rather a lot. "We didn't have the rights to the Dylan Thomas poems so she took his verse and reflected it - few people have actually picked up on that.

"She's a very, very talented writer," she says. "It was fantastic to work on something that we've had so many conversations about. I wasn't that convinced that it was actually ever going to be made."

The irony is that it was the attachment of Knightley, with rocketing box-office appeal, that finally helped get the film green-lit. "It's an art house movie," she shrugs. "It's very difficult to get those kind of films made."

The Duchess, Knightley acknowledges, is a much more commercial film. It stars Ralph Fiennes as the Duke of Devonshire, the most powerful peer in England. He and his spirited teenage wife clash from day one - he's more interested in bedding the maids and talking to his dogs. Before long he's installed his mistress (Hayley Atwell) in his sumptuous houses. Georgiana, meanwhile, becomes the darling of society, a powerbroker amongst the Whig party and enamoured of a young, up-and-coming MP (Dominic Cooper). But can the young duchess escape her stifling situation and her overbearing husband?

The parallels between this Lady Spencer and the more recent Lady (Diana) Spencer are clear, although Knightley - perhaps speaking of her tender years - says: "Honestly, I don't know very much about the whole Lady Di thing."

In a career with more than its fair share of corsetry, bonnets and lacy undergarments, how does she account for the fact that she's had more success with period-set movies? "I don't know. You look at the contemporary films that I have had success with and it's been Love Actually and Bend It Like Beckham," she offers, meaning, I think, that those films were very successful indeed, which they were. "I've just been offered more interesting characters in period films for some reason than I've found in contemporary films," she adds. "I personally love watching period pieces. You can completely dive into another reality. If you're doing one set 300 years ago you can let your imagination go wild. I think I find it more freeing to work in period."

It seems to me that period films are a better fit for what is, at this point, Knightley's acting style. Her best performances have come in Pride And Prejudice and Atonement - in each, she was brilliant at conveying the characters' buttoned-up, coiled-tight natures. I put it to her that she is an old-fashioned actress - that is, not flamboyant or showy - and she's better at conveying detail and nuance.

"Eh, phew, um, yes " she falters, wary of being boxed in. "Not saying that I'm never going to do something that's completely overtly flamboyant."

A Tarantino?

"Oh that'd be great, but at the moment I am completely fascinated by the minutiae of emotion, as opposed to the big gesture And possibly being British, I'm fascinated with repression. Emotional repression, I f***ing love that shit," she beams. "Again, it's the contradictions of what you're actually feeling inside and what you present on the front. I find it fascinating to look into."

That said, for Keira Knightley costume drama doesn't just mean simmering, repressed passion. In the wildly successful Pirates Of The Caribbean series, which launched her career into the stratosphere, she was required to buckle her swashes alongside Johnny Depp and Orlando Bloom. In King Arthur, Jerry Bruckheimer's misfiring epic, she had to learn to loose arrows like a proper knight. She also had to suffer the indignity of having her less-than-buxom chest digitally enhanced for the film poster. How did she feel about that?

"I think I've got at least a sense of humour about it," she says. "It did actually make me die laughing. I have a picture of me next to the poster trying desperately to get as much as cleavage as that. You know, you sign on to do certain kinds of films, and you kind of expect that from a big old Hollywood blockbuster. And no, there wasn't anything I could have done about it. Would I have preferred to say: Do you mind not giving me double D?' Yes. The original poster, actually, that came back, was incredible - like, droopy tits. Like, enhance them, but don't give me your grandmother's tits. At least give me pert tits for f*** sake." She grins. "I did at least get some pert tits that weren't mine, but it doesn't really matter, does it?"

Indeed not. Especially when she is assailed with more damaging indignities, such as the repeated suggestion that she suffers from an eating disorder. Mostly she ignores these, although her mother recently accused the press of being no better than playground bullies for the way they picked on her daughter and her body. Nevertheless Knightley did sue (successfully) the Daily Mail for a false story implying she had lied about suffering from anorexia and linking her to the death from the disease of a young girl.

"That one was particularly horrendous. I don't read anything that's written about me, but my agent did phone me up and she said: I'm really sorry, there's been something that's written and it's the worst thing that I've ever read written about anyone. And you're going to have to do something about it.' "I'm not a particularly litigious person - I don't have the money to be suing national newspapers," she claims, "but I think there are lines that certainly shouldn't be crossed. And you have to be clear where those lines are. That story was just horrible. And if you're saying that I have a mental illness, then I'm obviously not equipped to work, so it would have people - insurance companies, whoever - thinking twice about hiring me. So it's also really trying to f*** up any kind of career that I have. So you have to take that seriously."

These things get conflated, I say. Stories about your weight, the enhanced boob size ...

"Wait a minute," she interjects sharply, "enhanced boob size means anorexia?"

No, I was going to say that those stories, plus your modelling for Chanel, in the eyes of certain sections of the media make you, somehow, fair game. You're using your body to sell stuff.

"D'you think they'd do that to a man?"

No, they wouldn't.

"No, that's interesting isn't it?" she says with a steely smile. "It does come back to the original point with The Duchess, about how we take women's bodies apart. Yeah, I find it fascinating."

Amanda Foreman, who wrote the bestselling biography upon which The Duchess is based, has said she is "amazed" by Knightley's ability to "keep her composure" given that, like Georgiana, she is exposed to the "intense glare" of the media. This echoes something James McAvoy said to me about his Atonement co-star. He described her as "the biggest star that Britain has", adding: "I didn't expect her to be so cool and so professional and so experienced." I ask Knightly about her "composure", this "cool", about coping with the same "intense glare" of media attention that Georgiana dealt with.

She replies by saying the duchess "grew up with it". The point being, I think, that Knightley didn't. "But when you go back to this incredible need for attention, there was a part of it that was hugely enjoyed I think that that's what was always going to happen with her. She was put into that society. She very definitely made it a million times bigger when she married the most powerful man in England and then became a political hostess. But she did that because she needed it."

She needed the oxygen of publicity?

"Yeah, and I took it as being a very sad reason. A very lonely reason, on the inside." Knightley is talking more quietly now, her sparkiness dulling. "With me it's, ah, very different "

I mention some recent newspaper pictures of Sienna Miller distraught in her car in Malibu as she sought refuge in a police station from a convoy of pursuing paparazzi.

"Yeah, annoying when that happens," she says drily. "I hate that. It's incredible that it's allowed to happen. I'm not going to get I could so easily rant," she exclaims, literally biting her tongue. She recently talked about how paparazzi will shout "whore" at her to try to get a picture of her looking upset. "No, it's incredible that it's allowed to happen."

Because our privacy laws aren't strong enough?

"No," she replies, meaning yes. "When you're talking about cars going very fast, and kids possibly stepping out in front of them - I think something should be done. Having been in several chases like that, somebody's going to get very hurt very soon."

The sad thing is, that's what it may take to end it.

"That's what it does take. That's what's terrifying. You do occasionally have to get police involved and I think it would be great if they had some power to actually do a little bit more about it. I can't even It's so " she smiles thinly. "But there you go," she says flatly, finally.

Keira Knightley doesn't want to say any more. She knows any expressions of weakness or upset or anger just encourage more newspaper headlines and scrutiny, that her life would be even more hectic. Her 18th-century duchess didn't have a terribly happy ending. Professionally and personally, Knightley is doing all she can to stop her life going in anything like the same direction.

The Duchess is released on September 5