Midway through Act One of Sister Act, the ingratiating musical that is currently on view at the Fox Theatre, a shy police officer named Eddie Souther (Chester Gregory, reprising the role he created on Broadway) imagines what it might be like if he were as cool as Tony Manero, the disco-dancing dynamo portrayed by John Travolta in Saturday Night Fever. In the batting of an eyelash, Eddie's basic blue uniform vanishes, and fantasy becomes reality. Or perhaps reality becomes fantasy. Whatever is going on, it occurs with surprising dexterity and is very clever indeed. Eddie's "how'd they do that?" costume change is as effective as anything you're going see (or hear) all night long. Thanks to unexpected moments like this one, the show charms. If you can't have a good time atSister Act, you're not trying.