"Pistol! No!" shouts Amber Heard as she's dragged by a beaded leash across Los Angeles' Pan Pacific Park. We're supposed to be having a quiet afternoon picnic, but Pistol, a two-pound teacup Yorkie with a cute pink bow on her head, has spotted a brown schnauzer type whose rear end must be investigated. "I'm really sorry," says Heard, 23, who's starring in the remake of The Stepfather, out this month. "My dog has decided she wants to go over there, and it's really hard to stop her once she's made up her mind."

You know what they say about dogs and their owners. In this case it's true. Heard grew up equally headstrong in Austin, Texas, more interested in reading Henry Miller and Ayn Rand than in sitting through Bible study with her Catholic-school classmates. She is now a devout atheist. "I did what I needed to do to be independent," she says after Pistol allows us to settle at a shady picnic table. "I never felt like I was telling the truth when I prayed. When you're growing up and one thing is cool and admired and another is distasteful, where does that put you if you side with the latter? It put me . . . out."

Or, to be more specific, in New York, where Heard moved at 16 to model. At 17, she decamped again, for Hollywood, where her mischievous-cheerleader looks helped land her a part in the 2004 film Friday Night Lights (it also didn't hurt that Heard's family is from the Midland-Odessa area, where the movie was set). That performance, and her casual confidence, led to a string of vixenish roles in movies like Alpha Dog, Never Back Down, and Pineapple Express, in which she proved her acting chops by pretending to be attracted to Seth Rogen.

The Internet Movie Database is blunt in its description of Heard's oeuvre—"often plays sexualized teenagers"—but you're more likely to find her holed up in the L.A. Public Library than in the back room at Winston's. As we slice bits of Cheddar for Pistol, Heard name-drops philosophers Omar Khayám and Baruch Spinoza. By contrast, when she signed on to Stepfather, she had to be told that her costar, Penn Badgley, was on a TV show called Gossip Girl. Heard admits to owning a .357 Magnum (hence the dog's name), but when asked what she does for fun, she launches into passionate odes to Christopher Hitchens and cooking with truffled cheese. "We'd usually find her off in a corner reading some piece of literature that most of us wouldn't understand," says director Derrick Borte, who worked with Heard on this fall's indie drama The Joneses. "She is worldly and wise beyond her years."