She was born in South Carolina and moved to New York—she'd visited only once before—in the late eighties, after graduating from North Carolina School of the Arts. The city has been home ever since. The director Joe Mantello, who was at School of the Arts with her, says that then, as now, she could seem "wispy and ethereal, but she's incredibly fierce, has incredible backbone, and really knows herself. She has this fragility, but on the other hand, don't fuck with her. "

"I was wildly uncultured," Parker says of her first years in New York. "I'd never seen a copy of The New Yorker. I didn't know what a Hirschfeld was until one was done of me. I was 24 years old and I was on Broadway. It's so cool when I think about it. I kind of want to step back in my body for a day and relive it.

Through the nineties, her star rose. There were film and TV roles, but it was as a first-rate stage actor that she established herself. Two particularly memorable performances were in Paula Vogel's How I Learned to Drive and in David Auburn's Proof, for which she won a Tony in 2001. Recently she's been working the small screen: in Angels in America and The West Wing. Now she's starring as a pot-dealing suburban widow in the new Showtime series Weeds. "I liked the premise. I think it's rich and interesting," Parker says. "I love the psychology of a woman who would do something like this."

"She created a whole profile for the character beyond what was written," says Jenji Kohan, the show's executive producer. For instance, Parker decided that her pot-dealing hausfrau, Nancy, drank a lot of coffee and had been a ballerina. "You never know what she's going to do," says Kohan, "and sometimes that can be nerve-racking. But she's golden up there."

"She's famous for not being able to hit a mark," says the playwright Craig Lucas, who secured her for his own Longtime Companion, Prelude to a Kiss, and Reckless. "So many stage actors have learned to 'turn in' a performance. Mary-Louise is categorically opposed to this kind of approach, which raises the bar for everyone else. You can always see her character thinking, thinking, thinking."

The brainy former ingenue is, remarkably, turning 41—which is prehistoric by Hollywood standards, unless you happen to be male. Yet Parker seems immune to the enforced downshift many great actresses endure, partly because she had the foresight never to have had much of a film career to begin with. (She didn't even get cast in the Hollywood versions of two acclaimed roles she created, for Prelude to a Kiss and Proof.) "If I were somebody who used to get A-list movies, it would have been a big shock to my system," she says matter-of-factly. "But there was nothing for me to lose. And I've gotten some of my best parts in the last five years."

One of them is a real-life role. Parker shows me a delicate gold necklace with a small W pendant—for Will, her 18-month-old son.

"Nothing tops it," she says of being a mother. "There's not a single moment that I haven't savored and loved—not that I haven't been tired. All my life I wanted a girl—I don't even know why—but now I just want 20 boys."

The inevitable question is posed about the departure of Will's father, Billy Crudup, just before the baby's birth. (Crudup, Parker's companion of many years, left her toward the end of her pregnancy, eventually taking up with Claire Danes. All three of them uncharacteristically landed in the gossip columns.) Parker, a reluctant and wary interviewee, doesn't bristle—though it remains a subject she's unwilling to discuss.

"Out of respect for my son's privacy, I'll decline," she says, not unkindly. "That's really why I don't say anything: It's not about me anymore. If it were about me, it would be a different story."

Given that Mantello, one of today's leading theater directors, is comfortable calling Parker "the finest stage actress of her generation," it's tempting to wonder what else she might take on.

"Like, what, doing Uncle Vanya when I'm 50?" she replies. "Or Shakespeare? No, I don't really like speaking in meter. I did a Jacobean play once, and I've done Molière, and I found it really restricting. I hate Molière.

Animated till now, she grows thoughtful.

"I always wanted to do plays that no one had ever read. I wanted to create new characters," she says. "And that's what I've done. It's the one thing in my life that's worked out perfectly." She laughs, with more than a little rue. "I can't buy the right cell phone, but I pick good plays."

And then, one could add, she knows exactly what to do with them.

"I was a quiet, really awkward child," Parker says. "But then I got on a stage and it was suddenly fine: This kid who was not fine was ... fine. And I still love getting out of my world, being on a stage and just going. No one can stop me, no one can pull the plug, no one can say 'Cut,' no one can turn out the lights. I hold the reins. It's my little Disneyland ride."