http://latimesblogs.latimes.com/awards/2011/01/how-the-first-animated-sequence-in-a-harry-potter-movie-came-about.html
There is a rather nice quote at the end of the article by the animation director, Ben Hibon, behind the sequence:
"Animation is the art of smoke and mirrors, lights and shadows, and the team of artists behind this piece are true magicians."
