Becoming ... Keira Knightley

To bulk up for her role as Guinevere in the Jerry Bruckheimer-produced King Arthur, Britain's Knightley (promoting the film on The Late Show with David Letterman in June) lifted weights and boxed two hours a day, four times a week. She also practiced archery, sword-fighting and horseback riding for up to seven hours a day before filming began.

She claims to be "more of a tomboy than a girlie-girl," but for major events, Knightley cleans up well. In December (left), she paired a lace-trimmed dress with a funky bowler; channeled Audrey Hepburn in a black strapless dress in May (center); and for a TRL appearance in June, paired a trendy tweed-and-lace skirt with a pretty blouse (right).

"She's a very smart, very bright actress," says King Arthur costar Clive Owen of Knightley (at the film's New York premiere in June). "The hugely impressive thing about her is that she's incredibly rooted, she's incredibly centered, down to earth."

Knightley, who was interested in acting but struggled with dyslexia as a child, learned to read at 6 after her mother promised she could get an agent if she did so. By summer's end, Knightley (pictured at 8) had scored an agent, and a year later, a role in the British TV movie Royal Celebration.

Knightley recently has been spending time with Irish model Jamie Dornan, 22, whom she met at a New York City photo shoot in August. Though she won't reveal if the pair (in May) are dating, when asked at a London event if they were a couple, she coyly told photographers, "Well, it looks that way, doesn't it?"

Naturally lanky Knightley trained with a professional coach for her role as soccer phenom Jules Paxton in 2002's Bend It Like Beckham, building muscle and jumping a dress size in the process. But that's not the only thing that resulted: "Everything's changed for me since Beckham, she says. "It's been rather mad, but lovely."

Middlesex, England, native Knightley says her parents – mom Sharman Macdonald (in 2003) is a playwright and dad Will Knightley is an actor – influenced her decision to become an actress. Growing up, "I spent most of my time around people within the industry," she says. "I think when you're around really creative people from a young age it does tend to rub off on you."

At 14, Knightley got her big-screen break as Sabe, the decoy to Natalie Portman's Queen Amidala in 1999's Star Wars: Episode 1 – The Phantom Menace.

Of her star-making role as headstrong Elizabeth Swann in 2003's Pirates of the Caribbean, Knightley (at the film's Los Angeles premiere) says: "It was great fun. I think there were certain bits when I was climbing up the side of a ship in a dress, heels and a corset at 4:30 in the morning that I wasn't too happy, but it was great fun."

"If the script calls for it," she'll do it – pose nude, that is, as she did in the 2002 remake of Dr. Zhivago. "I'm not going to say, 'I really love taking my clothes off, it's fantastic!' " Knightley told London's TimeOut this year, but she won't count it out.

When it comes to men, Knightley says she finds "humor attractive every single time, and talent, I suppose, is always very attractive." British actor Del Synnott, 26, must have fit the bill: After meeting on the set of the 2001 TV movie The Princess of Thieves, the pair (in 2002) dated for two years.

Producer Jerry Bruckheimer predicts that in 10 years, Knightley will be "a major star. She is a terrific kid. Not to mention her enormous talent. In the end, it all comes down to her screen persona. I mean, you like what you see onscreen; she becomes the character. She is the real thing."