Laura Prepon knows exactly how her late father would react if he knew she smoked. “He would probably kick my butt,” she says. Her dad, Michael Prepon, an orthopedic surgeon, died at 49 while undergoing heart surgery in 1993, and his two-pack-a-day habit didn’t help. “But my grandmother just died of lung cancer and never smoked a day in her life,” reasons Prepon, 19, who first lit up two years after her father’s passing. “Or maybe I say that to justify that smoking is okay. I always try to rationalize things to make the bad things I do okay.”

She happily makes no excuses for her turn on the hit FOX sitcom That ’70s Show, in which she stars as a Wisconsin teenager in the disco decade. “She has a Lauren Bacall quality to her,” says Mark Brazill, a co-creator on the show, on why she was hired. “She’s a woman of substance.”

Topher Grace, 21, who plays her neighbor and boyfriend, believes Prepon is as groovy as her character, Donna Pinciotti. “She actually is that girl that you never think exists, that quintessential girl that every guy is looking for,” says Grace, a frequent visitor at Prepon’s two-bedroom L.A. apartment.

Like Grace, Prepon has spent much of the last year learning about the Me Decade, brushing up on all things ’70s by studying reruns of The Brady Bunch and trying on those odd sartorial artifacts known as bell-bottoms. She still shudders over a recent ’70s outfit for Donna, a shimmery maroon spandex leotard and wrap skirt. “I’m so not into tight clothes,” says Prepon, who regularly buys pants in the men’s department for their baggy fit. “I almost killed wardrobe when they put that thing on me.”

Despite her aversion to girly duds (she owns one dress, a blue Calvin Klein she wore to last year’s Emmys), Prepon insists she is not a tomboy. “People think I am, because I don’t wear makeup and I wear my hair back,” she says. “If I have to get dressed up, I rise to the occasion.”

The tomboy perception is helped somewhat by her height—5’10”. But growing up in affluent Watchung, N.J., she was never teased about it. “Usually boys pick on girls,” says Prepon, who is currently not dating anyone. “But I was friends with all the boys.”

Her strong relationship with her family helped Prepon cope with her father’s death. After he died, Laura and her siblings—Stephanie, 29, an editor at Glamour magazine; Jocelyn, 27, a law-school student; Danielle, 25, an art student; and Brad, 21, a senior at Franklin and Marshall College in Pennsylvania—hugged him for a final good-bye. “They really felt his spirit was there,” says their mother, Marjorie, 58, who gave up being a high school teacher to raise her family. “That helped.”

Soon after her father died, Prepon, who had wanted to act since she was a child, transferred to the Professional Children’s School in Manhattan. Her mother shuttled Prepon to the city for classes and auditions daily. She finally landed a gig in 1995, kissing a frog for an MCI phone commercial, and later appeared on They Go On, one of the first series broadcast over the Internet, before That ’70s Show came her way.

Since living in the past can get tiresome, Prepon hopes to segue into films. So far, though, her inability to master the art of the audition has prevented her from breaking through on the big screen. “They always give you the hardest scene, where the girl breaks down and cries,” says Prepon. “But I never cry. It takes a lot to get me there.”

Grace, for one, has no doubts about Prepon’s future. “I’m sure she’s going to be the biggest movie star,” he says. “I hope she stays friends with me!”

Jason Lynch

Kelly Carter in Los Angeles