Some challenges the filmmakers would face would be showing how number four really feels because most of the parts of the book that represent how he feels about the situation he is currently in would translate into a pretty boring scene on the big screen. because most of them are him sitting there and thinking things over in his head and i don't think movie goers want to see someone sit there in silence thinking about things. one thing that the filmmakers must incorporate is when number four meets Sarah and there relationship throughout the story because that relationship really effects how the plot unfolds later in the book. Another thing they would need to keep is part where number four gets the mysterious letters and when the Mongolians show up. The third thing they would need to keep is all the other action scenes because that usually tends to make a good movie.
One thing the filmmakers should probably take out is most of the training that he does everyday. They should put a little of it in the movie to show the viewers what he is doing but only show it once not everyday like they do in the book. Another thing they should take out is when mark and his Friends jump number four and he fends them off because this does not really add to the story in any way except put a little more action in it. the last thing that they should take out is the long road trip number four and Henri make up to Ohio they should just show when they get there and not the hole drive like they do in the book.