Since the hit sitcom '3rd Rock From The Sun' ended, Kristen Johnston has built up a substantial theater resume in New York.

Johnston plays a messed-up mother in 'Scarcity' (l., with Meredith Brandt, Jesse Eisenberg and Michael T. Weiss).

The "this" she refers to is, of course, acting on stage. "I don't do it to 'up my profile,'" she adds. "I do it because I love it."

Johnston's "profile" is still pretty high some six years since the hit sitcom she co-starred on, "3rd Rock From the Sun," left NBC after five seasons, but as she admits, theater is a good way to keep it life-sized: Clothes racks are scattered around the room, and sliding curtains section off areas for privacy. Johnston's black pit bull, Stella - saved from a shelter - nestles nearby. And it all suits Johnston, svelte in blue jeans and a white blouse with eyelet trim, better than life in Los Angeles.

"I'm very proud of '3rd Rock,' and I was grateful every day," she says. "But it's the other stuff that came with it that was difficult." That included paparazzi who "were looking through my garbage and trying to follow me home. It actually scared the s- out of me," she says in earnest, clearly still a little spooked by the experience. After the sitcom ended in 2001, she landed the role of catty Sylvia in the Broadway revival of "The Women" and gladly returned to New York, where she had studied acting. She arrived here Sept. 10.

The next day clarified everything. "I was like, 'This is my city!'" she says. "I don't ever want to leave!" She's been building a solid résumé of theater work since.

In "Scarcity," which opened Thursday - on Johnston's 40th birthday, coincidentally - the actress plays a rural Massachusetts mother trying to hold her family together. "She married the hero of the high school [who's now] an alcoholic, and she dabbles a little in alcohol herself," she says of her character, Martha. "And she's living a life that she didn't want, but has no way of getting out." Martha's son, Billy (Jesse Eisenberg), has the chance to go to a prestigious prep school, and as his ambitions are nurtured by a teacher, a relative tries to pry Martha away from her boozy spouse.

"I feel for Martha, because I think that she's doing the very best she can," says Johnston, whose research consisted largely of sounding out Thurber - the play is partly autobiographical.

Johnston hams it up with French Stewart (r.) in a scene from '3rd Rock.'

Theater has always been a love for Johnston. Born in Washington, D.C., she was 3 when her family moved to Milwaukee. Her father, a Wisconsin state legislator, and her mother, a homemaker, discouraged TV, so she connected to the stage. "Milwaukee had a great theater scene in the '70s and '80s," she says. "That was my inspiration." At 17, she came to New York to study theater at NYU; she joined an acting program at the Atlantic a year later.

Since coming back in 2001, Johnston's range has gone from the dangerously right-wing Aunt Dan in a revival of Wallace Shawn's "Aunt Dan and Lemon" to the sharp-tongued Beatrice in "Much Ado About Nothing" in Central Park ("I love the challenge of making Shakespeare sound like, 'Hey, dude, what's up?'"). And last year, she took on a new play in London, "Love Song," with Cillian Murphy.

Johnston hasn't abandoned film or TV; earlier this year, she played Drew Barrymore's sister in "Music and Lyrics." "I was the second banana, where I love being," she says with a hearty laugh. "Drew does all the press, and I'm off on the sidelines making bunny ears over Hugh Grant!"

And she has been relishing life in New York, doing what she loves in the place she loves. "Once that thing happens in your brain," she says of the city's pull, "you're just here, forever."