From PEOPLE Magazine Click to enlarge
Sure, Keira Knightley can be disarming: making fun of her beanpole figure; cheerfully calling herself a "lazy cow"; and—in the ultimate Hollywood sacrilege—defiantly refusing to employ an entourage. Don't be fooled. The British actress knows how to play dirty. She dove into a grueling, three-month workout regimen to play a buff, butt-kicking (and blue-painted!) Guinevere in the action flick King Arthur. "She wanted the body that went with her character," says Richard Smedley, who created her two-hour, four-times-weekly workout routine (she also studied archery, horseback riding and sword fighting). "She's very single-minded. If you get to where she's got in the time she's done it, it doesn't happen by accident."

Such unshakable determination has served Knightley well. From the soccer fields of 2002's Bend It Like Beckham to last year's hits Pirates of the Caribbean: The Curse of the Black Pearl and Love Actually, her spirited performances have Hollywood charmed. And, oh yeah, she's still a teenager—though she often plays more mature. "She's 19, but you look through the lens, and she's a grown woman," says Arthur director Antoine Fuqua. "She's tough as nails."

Her appeal? Easy. "I would describe her as the most beautiful tomboy you will ever come across," says King Arthur costar Ioan Gruffudd. During shooting in Ireland, she bested her male costars in archery ("I totally kicked their arses," she says proudly) and downed Guinness with them at night—a forgivable departure from her high-protein, low-carb diet, which added notable muscle to her slim frame. "The guys had abs and pecs built into their costumes," says Knightley. "I had to come up with my own."

As for the guys in her offscreen life, she declines to discuss her nearly yearlong relationship with Irish model Jamie Dornan, 22, with whom she discreetly cuddled at Arthur's Manhattan premiere. A previous romance with British actor Del Synnott ended after two years in 2003. "I think it's important to make time for the people in your life who you love," she says on the subject, "and who love you back."

Knightley "is maturing beautifully," says King Arthur and Pirates producer Jerry Bruckheimer, who attributes her poise to her "very strong family life." The actress is tight with her stage-actor dad, Will Knightley, 58, and playwright mom, Sharman Macdonald, 53. On the set of Pirates, mother and daughter were "like two sisters running around together," recalls Bruckheimer. Growing up in London, Knightley demanded her own agent at 3 because her parents each had one. "That was the kind of precocious little brat I was," she says. She got her wish three years later, in exchange for reading intently to help conquer her dyslexia. She juggled school and work for the next decade—and appeared in her first blockbuster, albeit briefly, as Natalie Portman's look-alike "decoy" in 1999's Star Wars: Episode I—The Phantom Menace—until she turned 16 and her parents let her follow her dreams full time.

Realizing that her success could "completely disappear" at any moment, Knightley, who recently bought a London flat, is piling on the projects. She plays an alcoholic waitress in next year's thriller The Jacket, a Pirates sequel is also "definitely" a possibility. She may also play another action heroine now that King Arthur has turned her on to a major perk of the genre. "That will give me an excuse," she says, "to actually go to the gym."

Jason Lynch. Sean Daly and Michael Fleeman in Los Angeles, Ashley Williams in New York City and Courtney Rubin in London

  • Contributors:
  • Sean Daly,
  • Michael Fleeman,
  • Ashley Williams,
  • Courtney Rubin.