Anyone been to a Harry Potter open casting call? What should I expect?

Question:

Anyone been to a Harry Potter open casting call? What should I expect?

I'm attending an open call in london for a significant female role in the next film! It will be my first audition ever! yikes!!
What should I expect?
What happens at the last casting call for Hp? How can i make a good impression and stick in the casting directors' mind to up my chances?!
What did people wear to their last casting calls?
xThanks

Best Answer - Chosen by Asker: According to what I've read about the Luna casting call, you may not even see the casting director. They were divided into groups and only 2-3 people were picked out from each to get their picture taken and put on file, and even fewer were called back -- based on appearance -- to audition. It may be different for this one (Lavender Brown, yes?) but try not to set yourself up for disappointment.. just enjoy the fact that you get to have a once-in-a-lifetime experience of going to an HP casting call, and whatever is meant to happen will happen. Good luck! :)