Keira Knightley, fashion chameleon

“WHAT does it feel like to wear a corset?” asks Keira Knightley, looking amused. The actress rolls her molasses-brown eyes and starts to gasp as though she’s choking on a fish bone. “Your ribs are crushed in and you can’t get your breath. Oh, and if you get emotional, you just can’t calm down at all,” she says as her chest heaves. She takes a deep breath and adds: “You can certainly understand why women were known as the weaker sex.”

Not that she’s complaining, mind you. Knightley has come to understand how certain clothes make her a stronger actress. She appreciates the emotional strain of a mercilessly cinched waist and wig that towers 2 feet high. Or the physical discomfort of a 1930s bathing cap and a flannel swimsuit. What Knightley endures on-screen isn’t really all that different from the pain that comes with a new pair of 5-inch Christian Louboutin pumps. She just happens to get paid well for her suffering.

“For me, fashion is all about fantasy and creating characters,” she says, echoing the aspirational allure of every Miu Miu or Gucci ad. “It’s amazing to work with a costume designer to build someone from the ground up. You start with the socks.”

Or the silk stockings. At 23, Knightley has time-traveled her way through decades and decades of costume dramas. Her major period films include “King Arthur,” “ Atonement,” “Silk” and three rollicking “Pirates of the Caribbean” movies. She was nominated for an Oscar as the hand-me-down-clad Elizabeth Bennet in “Pride & Prejudice.” Her latest film -- “The Duchess,” which opened Friday -- has her gallivanting around the English countryside in monstrous chapeaus that could house a family of field mice. Think “ Sex and the City,” three centuries ago.

The film chronicles the tragic, scandal-seared life of aristocrat Georgiana Spencer Cavendish, who married a distant duke with a wandering eye. His neglect and betrayal drove the duchess to crave more and more public attention. “As her confidence grows, her wigs get higher and her corsets get tighter,” Knightley says. “She’s trying to say ‘I am fine’ through her costumes, which I find intriguing.”

Cavendish’s outrageous looks -- hats festooned with foxtails, ostrich-feather headdresses -- also sparked fashion fads. “The duchess was the first to start ‘pregnancy fashion’ by wearing that maternity gown with the crystals when she goes into labor,” says costume designer Michael O’Connor, who fashioned 27 looks for the film. “She set all these trends and then wouldn’t be seen in them again.”

The same could be said of the woman who plays her, this sylph with cheekbones that could shave ice. No matter how dated the looks, Knightley’s distinctive on-screen styles always manage to affect street fashion. Her knee-high boots and flouncy blouses in the Disney swashbucklers had fashion editors purring about “pirate chic.” The laser-cut green dress she wore in “Atonement” last year heralded a return to retro glamour on the red carpet. Even the taut abs she showed off in her 2002 breakout role in “Bend It Like Beckham” drove women to drop and do crunches.

“Pirate chic?” says Knightley, her eyebrows at an astonished half-mast. “No, really?”

Clearly, the London native is both tickled and repelled by her influence. “Please don’t ask me about my style,” pleads Knightley, who has never perched in the front row of a fashion show. Ironically, you won’t catch this month’s Vogue cover girl paging through a fashion magazine either. “I don’t think anyone should dictate to you what you should like or wear because it’s trendy,” she says. Her own style icons are sartorial surrealists such as Björk and Helena Bonham Carter, ladies who consistently get called out by the fashion police. “They are completely out there, which I love. You have to be brave to express yourself.”

Or else find a character that will do the trick. Just moments before, Knightley had catapulted into a hotel room in New York for this interview and to be photographed. She eyed the array of casual looks -- lots of denim, a few dresses and a leather jacket -- laid out on the bed for her cover shoot and then asked: “OK. Who do you want me to be?” In the end, she didn’t want to wear “those L.A. skinny jeans” and opted for baggy Diesel denim and a striped navy Miss Davenporte knit sweater. She suddenly looked like one of Jean-Luc Godard’s gamin leads. “I feel like being French today,” she said and barely glanced at her reflection before posing for the camera, then changing again.

Still, it would seem that her nonchalant attitude about her image -- she has been known to describe her look as “disheveled” -- is as deliberate as another starlet’s practiced pout. Knightley’s reticence to cleave to a certain look makes her all the more malleable on-screen and off. Last year, fronting for the design house of Chanel, Knightley posed wearing just a bowler in ads for the scent Coco Mademoiselle.

“People thought it was shocking because I was naked,” she says. “I thought it was wonderful and bold, like Coco Chanel.”

Nudity works for Knightley because she’s forever a visual tabula rasa -- ever ready to don a bustle or a flapper’s sheath. Plus, she’s obviously no fashion misfit. Knightley tops every best-dressed list and often minces down the red carpet in flawless Chanel couture, Valentino, and bold statement jewelry. No elitist, she has also been spotted in dresses and blouses by Topshop. She dresses the part, all right. She just refuses to keep playing the same role.

“It’s just amazing to me that you can define yourself by what you wear,” Knightley muses. “But I guess that idea of designing the person you want to be goes back to ‘The Duchess.’ ”

She wiggles her bare toes and frowns. “I would never want to say that this or that is my definite style, because I never know who I want to be. What if I wanted to be someone different the next day? Then what?”

monica.corcoran@latimes.com