Audio problem: When Noriko tells Kawada that the food that he cooked was good, we hear him reply that his father was a cook, but if you watch him in the background you can clearly see he is just sitting there, and his mouth is not moving at all.
Audio problem: When Chigusa attacks Niida after he shoots her with an arrow in the cheek accidentally, she stabs him in the back, they fall and roll to the floor, and then he tries to escape from her and begins ro run away, but she grabs him and screams "Bastard!!" However her mouth is completely shut.
Chosen answer: Kawada hacked the system, learned about the collars, and transferred voluntarily to the class he knew would be participtaing in both versions. The only difference is when he transferred: in the book, it was right after his win, and in the film it was just for the battle. Shuya never hacked anything in either the book or the film. As to why the change was made, I can only assume that, given the shortening of novels involved in film making, it's easier to make Kawada a complete stranger than a loner that the kids recognize.