Toni Collette isn't all that complicated. Sure, she has been nominated for an Oscar (The Sixth Sense) and Golden Globes (Muriel's Wedding, Little Miss Sunshine). But she's still basically an actress looking for work. "I've never had that kind of idea of where I wanted to go," she says. "I've always been swept off my feet by roles." That's exactly what happened with United States of Tara, which premieres Sunday on Showtime (10 p.m. ET/PT).

"When I read the script," she says, "it was like a juicy page-turner that was full of surprises, and it made me laugh out loud. It was very moving, and as soon as I finished reading it, I was like, 'Yep, I'm doing it.' " It was a move as clear and straightforward as her order at a restaurant. At an eatery in the Four Seasons Hotel, she smiles up at the waitress and says, "Water. Flat. No ice. No lemon. Simple." The 36-year-old Aussie might be rational and no-frills, but her Tara character is anything but. She's a wife and mother of teenagers. And she has dissociative identity disorder; when she gets stressed, she changes personalities. In the first few episodes, viewers see three alternate personalities: Alice (a perfect homemaker), Buck (a motorcycle-loving male) and T (a rebellious teen). It's Collette's first TV role, and before Tara, she'd never really given small-screen work much thought.

"I started out in theater and moved into film," she says. "Most television programs make you have to live in L.A., and I didn't want to do that. It's a massive thing to move away from your friends and family and drag your own family halfway across the world." But because Tara is a pay-cable show limited to 13 episodes, her family had to relocate from Australia for only a few months. She and husband Dave Galafassi have a daughter, Sage Florence, who just turned 1, and a band, Toni Collette & The Finish. (She's the singer/songwriter, and he plays drums.) Other things made saying yes to the dark comedy easy: It's produced by Steven Spielberg and Oscar-winning writer Diablo Cody (Juno). The series also stars Rosemarie DeWitt (Rachel Getting Married) as Tara's sister and John Corbett (My Big Fat Greek Wedding, Sex and the City) as her husband.

Collette says Corbett wasn't quite what she expected. "I thought he was going to be like the characters I've seen him play. And there's certainly that element to him, but he has such enthusiasm for life, and … his humor is infectious." As Tara hits the small screen, Collette finds herself reading scripts and looking for her next sweep-me-off-my-feet part. But "after Tara," she says, "other stuff pales in comparison. It's just such a brilliant, satisfying role. And it's going to take something equally satisfying to do it."