To most of the country, Joan Cusack is better known as the big-haired secretary who snapped her gum alongside Melanie Griffith in 1988's Working Girl, or the suburban Mafia princess who tormented Michelle Pfeiffer in the same year's Married to the Mob. For years Cusack, 31, has made a mark (she was nominated for an Oscar for Working Girl) stealing scenes with her appealing eccentricity. Now, in Addams Family Values, playing nefarious nanny Debbie Jellinsky, Cusack steals much of the movie with her perfect comic timing -- and plunging necklines.

Says the actress: ''I've never seen myself with so much cleavage.'' Not that she had any objection to playing a sexy siren for a change. ''I mean, I grew up an Irish Catholic girl, thinking, 'There's something wrong with sexuality,' '' says Cusack. ''Then I thought, 'Wait a minute. What's going on? It's normal!' '' Yes, even Addamses do it. In the current hit movie, Cusack sets out to marry -- then murder -- painfully shy Uncle Fester (Christopher Lloyd). ''My favorite scene was where I'm sitting in the car putting on makeup and watching the house blow up,'' Cusack says. ''That was so me.''

Cusack is clearly rooky enough to be a spiritual member of the clan. Consider those frenetic felines. ''I don't like cats,'' she says. ''Someone gave them to me. I'll keep them, but I don't like them.'' As for those hot- pink-and-yellow kitchen cabinets, ''I painted them myself,'' she boasts. Her upbringing in the Chicago suburb of Evanston seems traditional -- until you learn of her stint on the high school . . . water-ballet team? Says Cusack: ''I have no idea how that happened.'' Blame it on Cusack Family Values. Joan's mother, Nancy, a former math teacher, and her father, Dick, who writes film scripts and sometimes acts, are both liberal activists. They brought their brood -- which includes Joan's brother John, 27, star of Say Anything, as well as fellow actors Ann, 32, Bill, 29, and Susie, 22 -- to peace marches and soup kitchens and steered them away from competitive sports. ''It's all about winning,'' says Nancy disdainfully. She held toys in similar regard: ''If children get bored, that's okay, because out of boredom they're going to figure out what to do.''

What her kids often did was plot out plays they would perform on the front lawn. Eventually, Nancy enrolled all the kids in Evanston's Piven Theatre Workshop ''to help them express their feelings.'' Joan expressed hers so well that, at 16, she was chosen for a small part in the 1980 movie My Bodyguard, which was filmed in Chicago. She used the money to finance her studies at the University of Wisconsin. She dabbled in theater there and graduated in 1985 with a degree in English. Then Cusack headed for New York City, where she landed a regular spot on Saturday Night Live. Success has brought raves -- and other rewards.

Cusack won't say how much she earns, only that it's less than comparable male actors -- but more than anyone deserves. ''Actors,'' she says bluntly, ''are overpaid.'' Still, she admits, ''it's nice not to have to worry about money.'' Her greatest personal indulgence: buying her loft overlooking Lake Michigan last year. She may miss out on a role occasionally by not living in Hollywood, but, she says, ''in Chicago I don't have to get all dolled up or act like a movie star.'' Or date a movie star. ''Actors are not very stable,'' says Cusack. She prefers local guys -- like her new beau, 30-year-old attorney Dick Burke. Ultimately, she says, she would like to have it all: marriage, kids,