She's a little stiff at first, this not being her sort of thing, generally speaking. She had danced and strutted on stage to Material Girl, but that was a performance, and this...this is just sheer, exhilarating, silly JOY, and she's not used to that.

But she loosens up as he encourages her, and by the time he sweeps her into a waltz hold she's laughing again, moving with him, dancing with him, letting go and being joyous with him.