In A Guy Thing, Jason Lee, the former Kevin Smith slacker who has flirted with leading-man status ever since he played Stillwater's lead singer in ''Almost Famous,'' seems to have been bitten by a vampire who sucked out all his prickly charisma. You see the promise of stardom dribbling through his fingers.
To begin with, this former skate punk looks terrible in sweaters; they turn him into the kind of emasculated dork you don't want to spend 90 minutes watching find his courage. Lee's whipped middle-class schlemiel is engaged to control-freak yuppie Selma Blair, but he really loves her cousin, Julia Stiles. There's not much more to it. ''A Guy Thing'' is abysmal, one of those dumbed-into-the-gutter farces in which the hero keeps having to do things like crawl out a second-story window and onto a tree branch. Each PG-13 version of a ''tasteless'' joke is powered by the laziest of coincidences (in a drugstore, Lee buys crotch medicine -- and runs into his future mother-in-law!).
What's painful is the sight of Lee transforming himself from a hostile rebel into a tame, simpering lapdog. In ''A Guy Thing,'' he looks as if he's about to get on his knees and start begging, and what it feels like he's pleading for is mainstream acceptance. Surely there's a better fate than becoming just this month's face.