Harry Potter and the Goblet of Fire is one of those novel/movie pairs where you just can't rely on the movie to be able to discuss the characters and plot structures of the book, because, for the sake of time and concision, much is omitted in the movie adaptation. Unfortunately, the elements of The Goblet of Fire that are lost in translation are the same elements that make up the lion's share of thematic material in the book.
One prominent character in the book is completely cut from the movie adaptation. This is Ludo Bagman, the head of the Department of Magical Games and Sports. Thematically, Bagman is an important character because he serves as an extreme counterpoint for the bureaucratized Ministry men who follow and enforce the rule of law. Bagman, a retired professional Quidditch player and something of a celebrity himself does anything but enforce rules. He flouts precautions that keep wizards concealed from Muggles at the World Cup. He encourages underage gambling, and then when Fred and George win a big bet they place with him, he pays them in Leprachaun gold (which vanishes an hour after it materializes) and never even returns the real money they initially gave him for the bet. He is constantly trying to help Harry in the tournament, disregarding the explicit rules that his own department established (in order to protect his own interests, because he placed a bet on Harry winning the tournament). Through all of this, he serves as a red herring; throughout the book, Bagman remains a primary suspect for someone who snuck Harry's name in the Goblet and works for Voldemort. In reality, he's merely a gambling addict who loves sporting events and competition. In his absence in the movie, Barty Crouch Jr. is really the only suspect, and there is very little mystery. The suspense and mystery of the book are replaced with the dramatic irony of the audience knowing all along that it's Barty Crouch Jr. somehow pulling the strings.
Speaking of the Crouchs, Barty Crouch Sr. plays a much smaller role in the movie than in the book. In the book, Sr. represents the corrupting nature of power. His reputation for absolutely enforcing a zero-tolerance policy against associates of Voldemort is complicated by the fact that, after sending his son to serve a life sentence in Azkaban, he later helps him to escape from prison. This whole side-plot, with the information that Crouch was a close contender for Minister of Magic, is missing from the movie, which thus fails to explore bureaucracy and the tendency of absolute power to corrupt.
Finally, the movie completely cuts the themes of subjugation and oppression of non-human participants in magical society that manifest, in the book, as Hermione's activism for Elfish Welfare and her condemnation of anti-giant sentiments. Ham-fisted though it may be, there is an obvious effort in the book to address some of the inconsistencies in representation and enfranchisement of different magical races and species in the Harry Potter universe; this effort is completely absent from the movie. If anything, the movie exacerbates existing stereotypes; for instance, in the book, Beauxbatons is a coeducation institution. In the movie, the school is turned into an all-girls school, and its students become caricatures of delicate femininity and French culture.
While it may be true that the movie is faced with the challenge of communicating the plot of an over-700-page book, the fact remains that it fails to include many of the main themes of the novel. In other words, if you have to write an essay about Harry Potter and the Goblet of Fire, do yourself a favor and don't rely on the movie!
