Rachel McAdams has had it with grand romantic gestures, onscreen and off. After The Notebook, she's finished with Romeos who send daily love letters. After Wedding Crashers, she's over Lotharios who interrupt "I do's" to declare undying love and an end to their rakish days. It's not that she's above obsessive behavior. "Once I showed up on this guy's doorstep dressed as a sexy cake," she admits. "I wore this white dress and had my hair done up like streamers, and sparkles on my face. I was sucking on a popsicle."

But these days, her idea of fun involves cold hearts, not sweet treats. McAdams, puckish and petite, wants to play dark. In her new movie, The Family Stone, she leads a mutiny against her brother's fiancée, played by Sarah Jessica Parker. "It's a nasty part," says director Thomas Bezucha. "There was pressure to cast her in the ingenue role. She refused."

Sure, ruining Ms. Sex and the City's life may not have been her agent's idea of a great move after Wedding Crashers became her third-straight $100 million—earning blockbuster. But now McAdams, 29, is ready to take risks. She even spent a recent vacation traveling alone in Costa Rica—and got stranded in a nowhere town on her first night, bunking in what turned out to be a crack house. "Men were pounding on the door, screaming things I didn't understand," she says. "I was terrified. I vowed to go home the next day." Instead, she stayed three weeks and learned to surf. "You have to be brave. Acting is about being vulnerable and allowing the unknown to punch you in the gut. Then you spit out your reaction—regardless of what it is." She'll try anything now, but she's done with popsicles.