For the first time since Liz Taylor played yo-yo with her body weight, an international obsession is developing over a movie star's weight gains. Toni Collette, the spunky Australian who stars in the bittersweet Aussie comedy Muriel's Wedding, is the actress. The obsession is with her 13-stone gain - or the 20-kilo or the 40-pluspound increase, depending on the country where people are obsessing - just to film a flick.

The 22-year-old Collette, who has since lost the extra fat, is bewildered and a bit bothered now that Muriel's Wedding, a hit at the Toronto filmfest last fall, is set to storm North America (it opens in Toronto tomorrow). "We're all surrounded by it," Collette says of the cult of skinny, "so I don't blame anybody for it. But I want to shoot the person who started it. There just seems to be this so-called ideal image of perfection that everybody has to aspire to. It's all shouldn'ts. But there are no rules in life. That's the main thrust of this film: Don't be afraid of who you are. It's okay! You don't have to live up to anybody's expectations." The movie Muriel is an overweight, underloved young Aussie woman with an abusive father, hateful `friends' who reject her and an irrepressible need to be noticed. She dreams of marriage - with pathological intensity. And Collette adores her. "Muriel is not typical of what you'd expect a screen heroine to be like. She's imperfect. She's perfect because she's imperfect.

"It's just such an amazing journey. She starts off as one person who goes through so much and ... (she pauses) ... I was going to say she ends up being someone completely different, but that's the whole point - she ends up being more herself!" Even Muriel's darker side didn't turn Collette away. "I was really worried at first because, if you look at what she does objectively, she could appear to be a really awful person. Because she lies and she steals and she cheats and, because she's so self-absorbed in trying to be accepted, she treats the people who love her the most really badly. I knew that I had to make it clear why she was doing it. She didn't see it as lying. She saw it as escaping into this fantastical comfort zone."

Collette's performance - this is her first major starring role in film - has been generating raves. She auditioned for the role the very first day writer-director P.J. ("I'm not the Croc") Hogan opened up shop. Hogan was impressed but he still went on to see hundreds more. "I think I was afraid to cast Muriel," Hogan admits, "because, if Muriel didn't work, the film didn't work. I had to be absolutely sure." Collette didn't look right, and not just because she looked too svelte. Her eye and hair color was not what he imagined when he wrote the script. Eventually, however, he changed his outer image for the inner energy. "In a film, you can establish a character physically instantly. What keeps them going with a character is what is in here (he points to his heart), what is behind the eyes, and that's where Toni is fascinating."

For her part, Collette just chuckles, displaying her typical absolute confidence: "When I read the script, I knew I would be doing it! Because I think it's the most beautiful and extraordinary story." Her eyes have it.