Because the book is told in first-person and the movie is a third-person narrative, readers didn't get to meet some of the characters featured more prominently in the movie. For instance, the game maker - who plays a very limited role in the book - gets a lot of screen time. Wes Bentley (American Beauty) plays Seneca Crane as an overconfident genius who takes pleasure in setting up the games themselves.
His game, however, falls apart in both the book and the movie, leading to a conclusion that has two tributes emerging from the battlefield, not one. Very little is made of this in the (first) book, but in the film, the game maker faces a great punishment for his failures. It isn't until the sequel book, Catching Fire, that the fate of the game maker is revealed - but in the movie, he is left in a room with only poison berries to eat. In the same way that he set death traps for others to fall into, he himself is sent into his own trap and forced to die for his failures.