LENOX, Mass. — How good can a fair play be? Pretty wonderful, actually — if you cast it right. Theresa Rebeck’s “Bad Dates,” which opened Off Broadway in 2003 and has since become a regional-theater staple, is the story of a ditsy Texas waitress with 600 pairs of shoes who moves to Manhattan, goes on a string of increasingly unsatisfactory dates, falls in with a bunch of Romanian gangsters and finds love without doing hard time. On paper it’s a cleverly written, overly cute one-woman romcom — and so it remained on stage when I saw Julie White perform it at Playwrights Horizons six years ago. But Elizabeth Aspenlieder, a splendid stage comedienne whose zany acting is part of what makes Shakespeare & Company the best theater troupe in the Berkshires, has miraculously contrived to turn Ms. Rebeck’s modest little show into a poignant slice of urban life that also happens to be drop-dead funny.