Fresh from his turn as Prospero at New York's Delacorte Theater, Next Generation's Patrick Stewart now appears in another Tempestuous role: as a gay decorator in Jeffrey. The challenge, says Stewart, was avoiding stereotypes. ''I asked [director] Christopher Ashley to slap me if I did anything clichéd,'' says Stewart. ''I didn't want the character to be two-dimensional.'' Though the film, about a man and his HIV-positive love interest, will be aimed at gays, Stewart isn't worried about straying from his Trek persona. ''I don't choose roles wondering whether fans will be shocked or pleased,'' says Stewart, who adds that Jeffrey is ''a 20th-century love story'' that will appeal to a wide audience. Besides, Jean-Luc Picard fans should take heart: There's a new Trek movie coming, possibly by Christmas 1996. Says Stewart, ''The producers, writers, and I are working on a screenplay now.''