 Daniel Radcliffe (qv) came up with the idea that, as a gesture of  respect to a teacher that Harry most certainly looked up to, he  would wear a certain type of clothes that resemble the outfit worn  by Prof. Lupin in his lessons from  _Harry Potter and the Prisoner of Azkaban (2004)_ (qv), during his  teachings. The director liked the suggestion, so that became the  basis for his look during those scenes.