Errr... Im probably going to offend a small measure of people here, but here goes.
I would like to see a movie made, as long as its made well(as im sure many here would agree). But the people you are thinking of casting for parts are... no good whatsoever for this film. That is a generalised statement... some of them may be good in the parts allocated to them. But i really do not want to see half of them in this film at all - you want this film to be original? Then get new mostly unheard of actors, drama schools would be good, but why not, as is done for theater shows, have auditions for the parts? Obviously not completely open auditions but take a selection of actors who have some experience behind them. This would leave you with a fresh cast of whom very few will be associated with other films, and so their characters will be much more potent and untainted.
These are just a few thoughts of mine, many people may disagree, but i do some amateur dramatics work, and ive seen a fair few people i would class as having that special something - even at older ages, and would class some of them as better actors than those in films. Remember it requires more skill to put on a good show than to record seperate scenes for a film.
As for parts of the book that are most important... I'll get my copy of magician and make some notes of things that cannot be got rid of, as there are little scenes that i forget in detail, but are important to the story of the film
~Ed~