Ann Miller
Saw the first 45 minutes of the movie last night and wasn't impressed--I prefer the play, but that's a topic for a different board.
On topic, Miller's part in this film is of Essie, the wanna-be dancer who, despite her constant dancing, never improves (at least, that's so of the character in the play).
Meanwhile, Miller really IS a dancer, and since she made a fortune doin' it, couldn't really fake being bad at it. A poor casting choice IMHO--the part called for someone clumsier.