John Stamos has played a hunky teenager (on General Hospital), a hunky rock star (on CBS' short-lived Dreams), and a hunky uncle (he's starting his fifth season on ABC's hit Full House). But on Oct. 13 at 9 p.m., the 28-year-old dark-haired heartthrob will be slipping into a decidedly un-hunky role as a sadistic kidnapper and rapist on ABC's Captive, a TV movie based on the criminal career of Robert Knott, who is currently serving multiple life sentences in a federal prison for a 1988 crime spree in the Northwest. ''During filming,'' Stamos recalls, ''I stayed up till 3 or 4 in the morning to get that bedraggled look. I didn't wash my hair for three weeks. My costars could tell you that I didn't exactly smell great.'' At first, Captive's producers were skeptical that the squeaky-clean Stamos could play such an unsavory part. So Stamos campaigned to convince them, meeting the producers in a swank L.A. restaurant in his grubbiest outfit. ''I wanted to show that I'm not just this coiffed, motorcycle-riding, girl-chasing, rock & roll guy,'' he says. Now that Stamos has gotten the grunge out of his system, what's his next extracurricular activity? ''For the moment, I'm a homebody,'' he says. ''I've been on the celibate and sober tour for three months.'' Staying home Saturday nights, no doubt, to wash his hair.