Principal photography for Harry Potter and the Prisoner of Azkaban began at Leavesden Studios on Monday 24th February 2003.
One of the most complicated set pieces to create was the Knight Bus, which appeared in this film. The audience sees the bus drive Harry through the streets of London and, despite its three decks, this was not achieved through CGI. The bus was driven to London, which required a lot of planning because the bus was specially built using components from two double-decker buses, in order to create a triple-decker. Taking the height of the bus into consideration, the route into London had to be carefully mapped out - bridges were clearly out of the question!
This film marked the first time the Dementors were introduced. Director Alfonso Cuaron and Producer David Heyman experimented with different methods to make it appear as if the Dementors were actually floating. Initially puppeteers were brought onto set, to move large pieces of cloth in a water tank which created the illusion that the Dementors were floating. However, this movement was later created through visual effects.