'Do you talk by rule then while dancing ?' Darcy asks Lizzy.
While she may intend to tease Darcy out of his studied coolness by playful observations, there is a point to them.
Couples who 'stand up together in a ballroom were expected to make conversation- (as Henry and Catherine do in NA, Mr K. and Harriet in Emma) waiting in line , facing each other, before the moment comes to dance up 'the set'.

Perhaps I don't regard Elizabeth's charecter in such a negative way. I believe Lizzy maybe trying to press Darcy to admit dancing is meant to be an agreeable social pasttime.