Every single film based on a book diverges at least a little from its source. Even so-called "faithful" adaptations make changes for the sake of the visual medium or simply just to streamline hundreds of pages into a two-hour film. "Beautiful Creatures" fans who are purists should know that Richard LaGravenese kept the brunt of Kami Garcia and Margaret Stohl's characters and themes, but he also changed a whole heckuva lot.
Prepare yourselves "Caster Chronicles" fans, for some major differences, seven of which I've detailed, but others you'll have to see for yourself.
Don't read ahead if spoilers bother you, obviously.
1. Ethan and Lena's Connection: One of the central plot points of the book, which is told from Ethan's perspective, is that he and Lena are eventually able to communicate with their minds (more like a silent phone call than regular telepathy). Their connection is so intense that they'll spend hours talking this way when they're apart, and Ethan sometimes has to shield his thoughts (see "Lots More Kissing") when they're together. The movie makes absolutely no mention of Ethan and Lena's supernatural ability to communicate.
2. Amma and Marian Are One Person: A major deviation from the movie is that LaGravenese combined two characters into one. In the books, Ethan has a grandmotherly housekeeper who loves to boss him around and make fattening but delicious Southern dishes every day. She's also a "seer" that can communicate with her dead relatives. Meanwhile, Marian is his dead mother's best friend and current head of the Gatlin (and as it turns out, the Caster) library. In the film, however, Viola Davis plays Amma but the character is more like Marian except that she's both a seer and the Caster librarian.
3. Ethan's Father: In the book, Ethan's grieving father - Mitchell Wate, a writer who hasn't left the house since his wife's death - is a frequently mentioned character. Mitchell spends every day locked in his study supposedly working on his new novel. His emotional absence and hermetic tendencies really bother Ethan, who misses having a present and available dad. In the movie, he's never discussed or seen, even during a holiday dinner.
4. Lots More Kissing: Garcia and Stohl made sure to make Ethan super gentlemanly. Since Lena can read his thoughts, he suppresses his desire for her by thinking of sports and history. Despite all of their romantic tension, Lena and Ethan don't give into their feelings until far into the story. In the movie however, Lena and Ethan don't stay in the friend zone for long at all. They get together quickly and have several passionate make-out sessions that aren't in the book.
5. Lena's Necklace of Junk: A symbol of Lena's quirkiness is a jangling necklace with charms that look like junk to most people – particularly the mean girls who rule Gatlin's high school. But the charms commemorate various parts of Lena's past, and Ethan loves and even adds to it. Lena wears the necklace in at least one scene of the movie, but it's definitely not discussed like it is in the novel.
6. Ridley's Appearance: Emmy Rossum is obviously gorgeous, but she's way more glamorous and femme-fatalish than the Ridley that Garcia and Stohl described. In the books, Ridley has long blond hair with dyed pink stripes in it who wears barely there cut offs and tanktops and sucks on a lollipop when she's compelling men to do her bidding. Rossum's Ridley has short red hair (although it changes a couple of times), wears elegant (but still revealing) outfits and eats sweets but doesn't do the lollipop licking (sorry, guys). LaGravenese does add a scene that shows what happened right after Ridley was "Claimed" as a dark Caster, a moment that's not detailed in the book.
7. The End: The movie's denouement completely changes what happens between Lena and Ethan in the novel. In the adaptation, Lena wipes out Ethan's memory in order to protect him from her family's evil curse. Although he's present the night of her claiming, it's not really him but another character who's taken on his appearance. So prepare yourself for a much different ending that leaves things nicely set up for a sequel but also in a widely different direction than "Beautiful Creatures" the book.