It was the hardest part that I've played in forever,'' says Sarah Jessica Parker of her first major post-Sex film role. The ensemble comedy revolves around Parker's Meredith, a tightly wound, socially awkward businesswoman visiting her boyfriend's close-knit clan of touchy-feely New England snobs. Explains Parker, ''She's convinced they're going to hate her, and the whole thing becomes a very vivid self-fulfilling prophecy.'' The Emmy winner was most impressed with writer-director Thomas Bezucha (Big Eden). ''It's the first time I've been on any movie or television show in 30 years that not one actor asked for a line change. Not one comma, nothing,'' Parker raves. ''He really knocked us all out.''