Which brings me to the only real disappointment in the cast, Megan Mullally, as Frederick’s cock-teasing fiancée Elizabeth. Talk about high expectations: Mullally’s Karen on “Will & Grace” delivered a boatload of giggles during the show’s eight seasons. You would think that if anyone could step into the fright wig of the late, great Madeline Kahn, it would be Mullally. She’s no musical comedy novice, and in any case her singing voice isn’t the problem. Nor is she attempting a Xerox of Kahn’s performance or her unique speech patterns — that petulant baby-doll voice rising to a buzzsaw shriek when she loses her grip.

Mullally is at least trying to find her own way. She uses a nasal Locust Valley Lockjaw accent to turn her Elizabeth into the kind of madcap socialite that populated so many 1930s screwball comedies, and she certainly dresses the part. But her character makes no sense (book writers Brooks and Thomas Meehan share much of the blame here). She’s cold to Frederick, keeping her goodies from him until their planned wedding night, but hints that she sleeps around with any Tom, Dick or Spencer Wadsworth III in the Social Register. She should be isolated from the other characters (both because she’s a snob and because they’re in on the monster’s creation and she’s not), but she leads a dance number in her first appearance and then shows up with an entourage of servants. She catches Frederick and Inga far more in flagrante than in the movie, yet seems to forgive him for his fraulein fling almost immediately.

Yes, few of the other characters are much more than the sum of their accents and eccentricities — I realize this is a musical, and a Mel Brooks one at that — but this Elizabeth’s contradictions were annoying rather than comedically fruitful. Worse, they made me dislike not only the character but the actress playing her — a feat I would not have thought possible.

I think Brooks, Meehan, and Stroman built up this part, either to tempt Mullally (her TV work makes her the biggest “name” in the cast, especially to non-theatergoers) or to keep her happy once she was on board. The film used the character sparingly, to great effect: Kahn got one funny, character-defining scene near the beginning, then stayed offscreen until she provided the final, hilarious sex joke that climaxes the final third of the film.

It also doesn’t help that, with the exception of her post-coital song, “Deep Love,” Elizabeth’s other numbers are lame. Which brings me to the other problem with this show: its score, by Mel Brooks. Mel has admitted he’s less a true composer than a melody-maker, with music arranger/supervisor Glen Kelly turning Mel’s snatches into a full-bodied score.