his fall, you'll see Gwyneth Paltrow take on her biggest role yet - literally. The lithe actress has signed on to play a 300-pound woman in the Farrelly brothers' latest movie, Shallow Hal.   The premise of the movie is this: As a child, Hal (Jack Black) promises his dying father he'll date only beautiful women. He keeps the promise, earning the name Shallow Hal. However, one day Tony Robbins hypnotizes him to see women's inner beauty and he falls in love with generous and kind-hearted Rosemary, who happens to be 300 pounds.   Under the spell, Hal sees Rosemary's inner qualities manifested as external beauty (read a gorgeous thin blonde). But when the spell is broken and he sees Rosemary's real figure, he's forced to make a choice.   Paltrow plays both the overweight Rosemary and the thin illusion that Hal sees. How is it that one of Hollywood's most elegant actresses has embraced a role that might threaten her picture-perfect image? After all, Paltrow's striking beauty has undeniably helped her career.   She got her first big break when Steven Spielberg cast her as the young Wendy in the 1991 movie Hook. With her long blonde hair and willowy figure, she later cut the perfect figure of the unattainable ice queen in the adaptation of the Dickens classic Great Expectations. And she was a convincing choice to play the trophy wife of a wealthy investment banker in A Perfect Murder.   What would make Paltrow, who has consistently chosen to play elegant (if spirited) women in films with a classic sensibility - such as Emma and Shakespeare in Love - want to work with the Farrelly brothers? After all, the duo is known for such low-brow efforts as Dumb and Dumber and Me, Myself & Irene.   Perhaps a desire to extend her range as an actress? Let's not forget the progeny of Tony-winning actress Blythe Danner and television producer Bruce Paltrow (St. Elsewhere) has acting in her blood. She began her career at the age of five, making a walk-on appearance in a summer stock production at a festival in Massachusetts, where her mother spent the summers acting. And early on in her career, her gritty portrayal of Ginnie in Flesh and Bone in 1983 demonstrated her willingness to take risks.   Or maybe the move is inspired by an enthusiasm to try something different. She's already earned a Best Actress Oscar for her role in Shakespeare in Love. So why not wade into a broad comedy and slip into a role that's completely foreign to her everyday existence?   Whatever her motivation, Paltrow's willingness to appear as a ballooned-up version of herself shows a certain amount of courage. It may also turn people's perceptions of her on their ear. Just as Cameron Diaz's performance as an unattractive and unkempt pet-store owner in Being John Malkovich did for her fans and critics.   Indeed, Peter Farrelly explained, "She (Paltrow) has a real good attitude. She's not full of herself and was looking forward to making fun of herself." The Farrelly's ultimately decided to cast Paltrow as Rosemary because "she really does have an inner beauty and we needed someone who could play a different person, whos not as attractive as Gwyneth, but you fall in love with her nonetheless."   The challenge, of course, was how the super-thin Paltrow - who eats only macrobiotic food and exercises regularly - would portray an overweight and dowdy character? She couldn't exactly eat her way into the role as Renée Zellweger did when she packed on 20 extra pounds to play the title role in Bridget Jones's Diary. No. Think more along the lines of Eddie Murphy in The Nutty Professor. In this case, it was a fat suit to the rescue.   While filming the movie, Paltrow decided to take her fat suit out for a test run to see what it was like to move in the world as an obese person. Wearing makeup and the suit, she went into the lobby of a New York Hotel. "No one recognized me at all - I almost got emotional because nobody would make eye contact. It was disturbing and sad. I got a real sense of what it would be like to be that overweight," Paltrow told Entertainment Tonight.   That experience likely heightened her respect for her heavy body double on the set, Ivy Snitzer, and other overweight people. She was nervous Snitzer would be offended by the fat suit, but Snitzer turned out to be supportive.   Of the movie's message, Paltrow says, "It's so much more important who you are and how you conduct yourself as opposed to how you look." It's a sentiment that she now believes not just in theory, but through experience. - Deena Waisberg