Great acting is not about glamour, and nobody knows that better than Phylicia Rashad. The former Cosby Show wife grayed her hair and donned a housedress in last spring's P. Diddy-headlined Broadway revival of A Raisin in the Sun—and she wound up with rapturous reviews, record-breaking box office receipts, and a Tony award. (She's the first African-American leading dramatic actress to win the statuette. ''I was stunned,'' she says. ''I asked, 'What, nobody was ever nominated before?''') Her secret? Listen up, ingenues! ''I'm not afraid to lose the froufrou,'' she says. ''Acting's not about nose jobs and liposuction. It's about being truthful to what you're doing.'' So faced with another plum part the 285-year-old ex-slave anchoring August Wilson's Gem of the Ocean the 56-year-old actress knew just what to do. She packed her fine figure with padding, slowed her elegant walk to a putter, and aged centuries under a shock of silver hair. And don't even think about calling Aunt Ester anything less than gorgeous. ''I think,'' says Rashad proudly, ''she's the most beautiful woman I've ever played.''