By Jim Farber
Updated January 11, 1991 at 05:00 AM EST

Any movie addressing a subject as serious and controversial as AIDS can easily be undone by good intentions. Longtime Companion covers the past decade of AIDS devastation in the gay community in fictional form — something no other theatrically released movie has so far dared attempt. Too bad the moviemakers were so intent on delivering their information-heavy message to a straight audience that they erased nearly every quirk and blemish of humanity that would make us care about the people affected. The characters here give off the same constipated, mannequin-like vibe we’re accustomed to enduring from TV- ; movie portrayals of gay men. In fact, the hero is distressingly similar to the protagonist in network TV’s first AIDS-related movie, An Early Frost — he’s little more than a handsome cipher. C-