Once you're face-to-face with Michelle Monaghan, you know exactly why Tom Cruise needed only 20 minutes with her to be convinced she was perfect for Mission: Impossible III. She's a classic hand-waver, barely able to restrain herself from grabbing your arm or leg to emphasize a point. For her, coffee can be overkill: "It will put me through the roof!" She laughs loudly, sprawled out and taking up more than her fair share of a booth at the trendy Mercer Kitchen in New York. Had she been on Oprah, she too might have jumped the couch. "I guess maybe to the outside world, it may seem odd," she says. "But the guy has the energy of 10 men. He's absolutely enthusiastic and really sincere. And it's a pleasure to be around, because I consider myself to be pretty enthusiastic as well."

Having put in just five years of acting before scoring a front seat in Cruise's diesel-powered M:I machine, the 30-year-old has reason to be joyful. But even if she's broken into Hollywood, she'd rather rhapsodize about her home state of Iowa and its "big, big sky, the bluest of blue, with really green crops and the sun beating down." Her hometown, Winthrop, is so small, she says, that when she was a teenager, the big Friday-night activity was cruising in pickup trucks in the nearest town with a strip, seven miles away. "Nothing happened," she says. "But we'd get home and be like, 'Oh my God, that was awesome!'"

Winthrop doesn't have a stoplight, let alone a Cineplex, so these days, if her childhood neighbors want to see her work, they've got an hour car ride to Waterloo. They made the trip to see the harrowing scene in North Country in which Monaghan's male coworkers knocked over a PortaPotty with her locked inside, just like they did to watch her save Val Kilmer's life and strip out of a Santa suit for Robert Downey Jr. in Kiss Kiss Bang Bang. (Her dad and two older brothers covered their eyes.)

Everything about Monaghan is so time-warpedly sunny that it all begins to sound like a setup for that age-old horror story of the small-town innocent who makes it big and has her personality sucked away and replaced by that of a power-hungry, manic-depressive vixen. But Downey doesn't think she's the corruptible type. "It drives me crazy when I see some people one year and they're on the rise, and then you see them the next year and now they've supposedly arrived and they've completely altered their personality," he says. "The great thing about Monaghan is that I don't see her turning into a basket case like half the fucking chicks out there. She's not a phony."