She has enchanted the Paul Harvey set at Quality Hill Playhouse for years now, but Karen Errington's charms are too rarely paraded before the rest of us. With her ace comic timing, her broad and buoyant voice and a bottomless bag of expressive glances, Errington makes each performance like one of those cardboard cornucopias that elementary school teachers tape up: The bounty just spills from it. Playing an agoraphobic trailer bride whose husband has been cheating, she brought an emotional heft that kept the Unicorn's light The Great American Trailer Park Musical from drifting into pure silliness. Her finest moment was her biggest, not because she needs to belt to win us but because she knows that singers have to build to the belting. Singing "Flushed Down the Pipes," she started out teensy and hurt but wound up swallowing the universe.