Keira's Quest(TIME)

A good line of dialogue can make a career. At just 19, Keira Knightley owns one widely quoted multiplex moment. In last summer's Pirates of the Caribbean: The black-toothed marauder with a metal pole and the trailer-ready line, "You like pain? Try wearing a corset." Now, if all goes according to plan, she will add a second epigram to her name. In King Arthur, which hits theaters nationwide on July 7, she plays Guinevere as a heavily muscled, lethal woman warrior. (Arthur, like Pirates, is a Jerry Bruckheimer production.) When one of the more sensitive knights of the Round Table confesses before battle to fearing the hairy, scary enemy, Knightley's Guinevere scoffs, "Don't worry, I won't let them rape you."

With a bit of tweaking, both lines could have been delivered by Arnold Schwarzenegger, which may explain why Knightley has so quickly ingratiated herself with moviegoers — and also why she doesn't consider it much of an achievement. Knightley, a native Londoner, has a refined look, brisk comic timing and a brawler's instinct for knowing when to shut up and throw a punch, but that doesn't mean she's ready to tackle Chekhov. "I don't think I can call myself an actress yet," she says. "I just don't think my skill level is that high. I hope that with every job it gets better. But until I'm good, I can say I'm trying to be an actor, but I don't think I've completely made it."

Knightley isn't being modest. She just doesn't believe in kidding herself. Of the acting challenge presented by King Arthur, she says, "I had to work out physically quite a bit, but pretty much it's scream a lot and enjoy being painted blue." (Her Guinevere wears so much blue war paint that she looks like the world's most ferocious Smurf.) Knightley is similarly dismissive of her breakthrough role in Bend It Like Beckham and her beatific cameo in Love Actually. Few actresses talk as frankly about the artistic limits of their work or as exuberantly about their physical imperfections. (Knightley likes to point out her acne for those who can't see it.) Few actresses also refuse to hire a personal assistant on the grounds that it would feel "absurd" to do so. Yet all this self-deprecation may explain why she's a burgeoning star. "She really does lack pretense," says Bruckheimer. "I'm a big believer that if you project somebody on a 100-foot screen, you look into their soul. Keira appreciates everything she gets, and she doesn't take herself seriously. Believe me, the audience sees that. Honesty and believability make movie stars. She's got both."

By sunnily questioning her talent and not approaching each role with a cyborgian ambition to be the next Julia Roberts, Knightley has separated herself a bit from the ever expanding galaxy of post-adolescent It girls (see box) — and staked a slightly more credible claim to actually being the next Julia Roberts. Knightley has Roberts' angular frame, avenue-wide smile and unforced sass, and she's grateful for what she calls the "insane and ridiculous luck I've had getting these big roles," but she does not possess the genetic code to be happy as a full-time romantic heroine — pirate thwacker. (To date, she is a holdout from the Pirates sequel.) What she would rather do is "keep learning, do strange things, keep pushing to get better."

That may elicit groans from the stadium seats, but it probably gets sustained applause in the Knightley household. Knightley portrays her parents — playwright Sharman Macdonald and stage actor Will Knightley — as the noblest and humblest of theater folks. One of her favorite stories about how they suffered for their art has to do with her conception: "I was a bet. My mum was desperate for another child, and my dad told her that the only way they could afford to have one was if she sold a play. So Mum wrote When I Was a Girl, I Used to Scream and Shout" — which had an eight-year run on London's West End — "and they got me."

Mom and Pop are not quite so humble in real life as they are in their daughter's mythology, but as Knightley watched her parents and her elder brother Caleb struggle through the uncertainty of life in the theater, she picked up a few critical lessons. First is the old adage that work is work, meaning that every job is a cause for celebration. "That's why my five-year plan is to take every job I can," Knightley says. "I know for a fact the work is going to dry up, and people will get bored of me. That's not bitterness, just the truth." The second lesson is that acting matters. "There was a sense that my parents' work was important and that it could change the world in a way," Knightley says. "That's an amazing thing to be around. It's inspiring. It makes you want to be great."

And she has wanted to be a part of that greatness from the time she was a toddler. Knightley was only 3 when she announced that she wanted an agent. Her parents persuaded her to wait a bit. But when their daughter's desire resurfaced a few years later, they reluctantly allowed her to act in television and commercials but not onstage, fearing that evening curtains would ruin her schoolwork. They also refused to give her a lick of formal training. Other than a recent Christmas gift from her father — a book on acting for the stage — her parents have been steadfast in their conviction that she should find her own way.

The result is that Knightley has the ethic of an artist and the unaffected energy of an autodidact. "Sometimes I put my head into a character's head and go really simplistically and think, like, What's the character's favorite color?" she says of her attempts at technique. "But I don't see how that helps so much." She has also tried listening to loops of Jeff Buckley and Nirvana to get into the right frame of mind to play an alcoholic Vermont waitress, opposite Adrien Brody, in the recently completed independent film The Jacket. "Oooh! I tried a bit of Method for that as well," she says in elaborate self-mockery. "The character was meant to be a bit of an insomniac, so I tried to not sleep, though it didn't really work. I thought about staying awake so much that it sent me to sleep. I don't have it down to a science yet."

What she does have is an obvious screen presence and an ambition beyond fame. "Nobody ever believes me when I say it, but that was never one of the reasons I wanted to act," says Knightley. "That's not what my mom and dad are, and my knowledge of the business wasn't anything to do with that." She recognizes that movie star is "a pretty good job," and she may yet sign on to that Pirates sequel. But asked if she's holding out because she would rather do something deeply obscure and conspicuously artistic, like, say, repertory theater in the British Midlands, she says, quite seriously, "You know, I might like that, actually. Could be quite fun."