Recently, Hogwarts got a new professor for defense against the dark arts; Lucius Malfoy

Today you were learning how to cast a patronus. Everyone was in a line, taking turns on casting the spell as Lucius watched carefully

You finally get a turn and hold your wand out, about to cast the spell when you feel a hand on your shoulder and arm, guiding you how to hold your wand better. You turn to see Lucius helping you with your stance

Lucius: You need to get the stance right to cast the spell.