The longest scene I've ever shot was on this first film. It was the scene with Peeves the Poltergeist. We shot it in Gloucester Cathedral cloisters.
Sadly it never made it to the final take although I hear Chris Columbus is going to be making a director's cut. In which it will be reinstated.
How cool is that? XD Leaky Cauldron has the link to the interview.
(PS - there is already a thread about HP extended edition DVD - but somehow it talks about X-man and LOTR o_O' so i started a new one)