Nobody wanted the lead in Muriel's Wedding, the latest quirky Australian film that uses ABBA's '70s superpop. (Priscilla was first.) Director P.J. Hogan says some actresses were actually offended that he asked them to play chubby, uncool Muriel Heslop, who sits in her room listening to the swirling disco of ''Dancing Queen'' because it's as close as she can get to someone sweeping her off her feet. Then along came Toni Collette, 22, who understood Muriel's satin-swathed fantasy. ''She tries too hard and it always goes horrifically wrong,'' says the Sydney resident, whose only other film credit is 1992's little-known Efficiency Expert with Anthony Hopkins. ''Muriel could easily be made fun of. I didn't want to trust anyone else with (her).'' Playing a misfit meant taking on the trappings of loserdom-a stringy haircut, an ill-fitting wardrobe, about 40 doughnut-stoked extra pounds, and a seven-week diet of ABBA Gold emanating from a sleek pink tape player on set. Over a year later, in New York for the U.S. premiere, Colette's transformation back to chic actress is startling. Fresh from filming Cosi (yes, another quirky Aussie comedy, this one with opera round-the-clock), she is thin again and chewing on fame. ''Six old men were hanging around outside my hotel last night,'' she reports incredulously. ''They wanted my autograph.''