Joannie Brosseau can sing. She can dance. She can play a wide range of roles, most of them with an irrepressible comic edge. Most of all, she has an indefinable quality that makes audiences snap into high alert whenever she steps on a stage. You could call her perky, but perky sounds mindless. Can you be smartly perky? That's certainly what Brosseau was in Boulder's Dinner Theatre's 42nd Street, bringing all her shine and energy to the part of Maggie Jones, kicking the action into high gear and encouraging her chorines to "Go Into Your Dance."