None of the cast knew how to dance.
Of course, we all know how to shake our booty, but formal dancing is no longer part of the core curriculum in most schools [read: none]. To prepare the young actors for the Yule Ball scene in Goblet of Fire, they all attended three weeks of dance classes. At the time, they had scheduled Daniel Radcliffe to the hilt with scenes and schooling so he couldn’t afford to attend all the dance classes. For this reason, filmmakers only captured him from the waist up. What his legs are doing is anyone’s guess.