There's something strange about American Gothic, the eerie drama coming to CBS this fall. No, it's not the Twin Peaks-meets-The X-Files-style plots, but that the show's writer and producer is Mr. Hardy Boy himself, Shaun Cassidy. ''He's an incredibly talented writer,'' says star Gary Cole (The Brady Bunch Movie). ''Some people might not expect him to be the source of this show, but why not? Whatever image he had is gone.'' The series, about a town sheriff who could be the very definition of evil, is not Cassidy's first tale from the dark side. He penned the 1991 USA Network thriller Strays, about a band of killer kitties. The 36-year-old ex-teenybopper says he's just exploring his options. ''Gothic is a very different type of show,'' says Cassidy, ''but I like taking chances like this.''