Continuity mistake: When Yuka Nakagawa is poisoned, her face lands in the bowl of poisoned spaghetti. A little later as the girls begin shooting each other, her face is now out of the bowl, and on the table instead. There was no movement to suggest that her face fell out of the bowl, as it quite clearly happens between shots (just before the girl jumps ono the table.)
Continuity mistake: When Shuya finds Utsumi dead, the proximity of her head to the girl behind her changes between shots, as does the position of her tie and gun beside her.
Chosen answer: He speaks in the book, actually, such as explaining that he flipped a coin to decide if he would participate in the game. He never felt emotion because of damage to his brain before he was born, which is why he's such a psychopath, but he didn't volunteer for the game. He was just part of the class that got selected, and had been forever. He actually had his own gang, who he kills at the beginning of the book.