There are plenty of vile, heartless thugs running around Prison Break's Fox River State Penitentiary, but none comes close to skeeving us out like Theodore ''T-Bag'' Bagwell, the drawling white-trash racist whose do- or-die rivalry with fellow inmate John Abruzzi came to a shocking head last week. Robert Knepper a veteran of TV and more than 20 films (Hostage) who's also starring as Joe McCarthy's henchman in Good Night, and Good Luck says he's having a ball being so bad, though he admits, ''I was concerned that I would have to call Fox and ask for some security. I thought I'd have a bunch of brothers coming up to me really angry about what my character does.'' Not so much. Excited fans approach him all the time to express their off-kilter admiration for T-Bag, a testament to Knepper's skill at playing slimy. ''People looove to hate me. I never play him like a stereotypical racist or redneck. He's actually quite cunning and smart.'' But will that help him when and if the Gang of Seven escapes in the season's fall finale (Nov. 28 at 9 p.m.)? ''I don't worry about that, because he has the ability to come up with some amazing shanks,'' he says with a nasty giggle. ''The audience knows he'll get away with it. But the question is how. And that's what makes it so fun to watch.''