So you read the book before you saw the movie. Congrats! Unfortunately—as these examples prove—that doesn't always make you an expert on what, exactly, is going to unfold on the big screen. (It should go without saying, but this article contains spoilers—lots of them. You've been warned!)
1. Jurassic Park
Jurassic Park, one of the most popular summer blockbusters of all time, doesn't completely line up with the events described in Michael Crichton's best-selling novel of the same name. At the end of the book, the Costa Rican military comes to the rescue by bombing Site A on Isla Nublar. But director Steven Spielberg felt like changing it up. Instead of a military intervention, Spielberg decided to have the T. Rex return to save the protagonists from a Velociraptor attack. "I think the star of this movie is the T. Rex," Spielberg explained at the time. "The audience will hate me if the T. Rex doesn't come back for one more heroic appearance."
The book and movie's body counts vary, too. By the end of the novel, John Hammond has died, and it is implied that Ian Malcolm has as well. Both survive in the movie. On the other hand, the park's game warden, Robert Muldoon, and IgGen's attorney, Donald Gennaro, perish in the big screen adaptation, but live on in the book.
2. Planet of the Apes
Planet of the Apes features one of the most iconic twist endings in movie history: Astronaut George Taylor (played by Charlton Heston) discovers he has been marooned on a post-apocalyptic Earth the entire time. But in La Planète des Singes, the French novel it is based on, the main character—journalist Ulysse Merou—lands on a different planet during the course of his travels, one inhabited by self-aware apes, sentient monkeys, and tribes of dimwitted humans. When Ulysse finally makes it back to Earth, he is shocked to learn that it is now 700 years in the future, and that a similar hierarchy has emerged at home.
Twilight Zone creator Rod Serling, who co-wrote the film's screenplay, was the one who ultimately decided to make the planet of the apes Earth in the distant future.
3. A Clockwork Orange
Stanley Kubrick based his screenplay on the shortened American version of the British novel by Anthony Burgess. This telling omitted the final chapter of the book, focused on Alex after he is rehabilitated. Though he grows out of his murderous tendencies in Burgess' text, in Kubrick's interpretation, Alex remains as psychotic as ever. Kubrick didn't like the tale's original ending; he felt it was entirely too optimistic given the story's tone and themes. "I think whatever Burgess had to say about the story was said in the book,” Kubrick said. “But I did invent a few useful narrative ideas and reshape some of the scenes."
Burgess was not a fan of the final product. "The book I am best known for, or only known for, is a novel I am prepared to repudiate: written a quarter of a century ago," Burgess later recalled. "It became known as the raw material for a film which seemed to glorify sex and violence. The film made it easy for readers of the book to misunderstand what it was about, and the misunderstanding will pursue me until I die."
4. Fight Club
The film version of Fight Club remains faithful to author Chuck Palahniuk's original plot—until the very end, that is. The movie version wraps up as the narrator, standing beside Marla, watches a series of explosions caused by his now-absent alter ego, Tyler Durden. At the end of the book, however, the narrator wakes up in recovery from his gunshot wound. He thinks he's in heaven, but Palahniuk makes it clear that he's actually in a mental institution. Several hospital attendants ask him when he's going to start Project Mayhem again, inferring that Tyler Durden is still very much a part of him.
Director David Fincher explained his choice by arguing that the book was too devoted to the narrator's alter ego: "[I] wanted people to love Tyler (Durden), but I also wanted them to be OK with his vanquishing."
5. The Scarlet Letter
Nathaniel Hawthorne’s novel The Scarlet Letter is an exploration of guilt, punishment, and mob mentality in 17th-century New England. At the end of the classic tale, the townspeople persecuting Prynne learn that the father of her baby is Reverend Dimmesdale, who eventually dies from immense guilt.
The 1995 film version of The Scarlet Letter opted instead for a happy Hollywood ending (read: no one dies). Instead, Reverend Dimmesdale and Hester Prynne leave their town in order to build a new life together.
6. Breakfast at Tiffany’s
Truman Capote's beloved novella was also given a simplified and sanitized Hollywood ending. In the book, Holly Golightly loses her cat and abandons New York for Argentina—it's unclear where the free spirit will end up next. The movie, on the other hand, ends with Audrey Hepburn's Holly reuniting with Cat and sharing a passionate kiss with neighbor Paul. (There's no romance between them in Capote's version.)
Capote wasn't a fan of the movie based on his work, nor of the casting of Audrey Hepburn. "I had lots of offers for that book, from practically everybody," Capote said in an interview. "And I sold it to this group at Paramount because they promised things, they made a list of everything, and they didn't keep a single one."
7. My Sister's Keeper
Jodi Picoult's My Sister's Keeper tells the story of a young leukemia patient named Kate, whose parents conceive another daughter, Anna, in order to have an organ donor for their firstborn. When she turns 13, Anna is asked to donate one of her kidneys to her dying sister. She refuses and sues her parents for medical emancipation.
In the book, Anna gets into a terrible car accident, and her kidneys are posthumously harvested for Kate, who survives. But for the 2009 adaptation, director Nick Cassevetes chose to reverse the sisters' fates. Kate ends up succumbing to her illness after she refuses to accept her sister's organs. Cassevetes believed his movie's ending was more accurate after he visited pediatric hospitals and talked to terminally ill patients.
"Going and visiting people in the hospital, this story repeated over and over and over again," Cassevetes told About.com. "In reality, none of these stories ended like the book did."
8. The Mist
Stephen King's The Mist ends on a vague note—a few survivors head towards the source of a mysterious radio transmission as the titular mist creeps around them. But director Frank Darabont decided to give the film a more definitive—and more gut-wrenching—conclusion. David, played by Thomas Jane, comes to realize that the group's survival efforts are futile. To prevent any further suffering, he kills the remaining survivors, including his son, just before the military shows up to inform him that the situation is now under control.
"How primitive do people get?" Darabont said of his new ending. "It's Lord of the Flies that happens to have some cool monsters in it." King, for his part, gave the new ending two thumbs-up: "The ending is such a jolt—wham! It's frightening. But people who go to see a horror movie don't necessarily want to be sent out with a Pollyanna ending."
9. The Lorax
At the end of Dr. Seuss' The Lorax, the tale's Once-ler gives the boy the last-ever Truffula seed in the hopes that he'll be able to grow a new forest. But there's no room for ambiguity in the story's 2012 cartoon version: Before the credits roll, new Truffula Trees are flourishing and The Lorax has returned to the forest.
10. Red Alert
Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb, based on Peter George's Red Alert, takes a comedic approach to the source material. Instead of narrowly avoiding a nuclear catastrophe at the zero hour (like the book does), Stanley Kubrick decided to blow up the world because of some petty bickering.
Originally, Kubrick planned to have everyone in the situation room get into a big pie fight. But "I decided it was farce and not consistent with the satiric tone of the rest of the film," he said.
11. Forrest Gump
There are some pretty major differences between Forrest Gump's book and film versions. Though the movie ends with Jenny's death and shows Forrest raising their child alone, the book wraps up with Forrest starting up his own shrimp business, in memory of his college friend Bubba. (Another key difference: in Winston Groom's book, Jenny survives, but marries another man and has his child.)
"[Screenwriter] Eric Roth departed substantially from the book,” Zemeckis told the Chicago Tribune. “We flipped the two elements of the book, making the love story primary and the fantastic adventures secondary. Also, the book was cynical and colder than the movie. In the movie, Gump is a completely decent character, always true to his word. He has no agenda and no opinion about anything except Jenny, his mother, and God."
Groom believed that the movie "took some of the rough edges off" his beloved character. In fact, he was so unhappy with the film that he started the book's sequel, Gump and Co., with Forrest telling readers, "Don't never let nobody make a movie of your life's story."
12. Who Censored Roger Rabbit?
Who Censored Roger Rabbit?, the inspiration for Who Framed Roger Rabbit?, is a surprisingly dark murder mystery. In the novel, Roger hires Detective Eddie Valiant to figure out why Rocco DeGreasy, the man who has the cartoon rabbit under contract, hasn't given him his own comic strip. During Valiant's investigation, Roger Rabbit is murdered and his wife Jessica is framed. Valiant spends the rest of the story trying to figure out who killed Roger. (The book ends with the revelation that a mysterious genie is the culprit.)
Although there's still a murder at the center of the 1988 movie version—this time, Toontown owner Marvin Acme is the victim—Disney and Touchstone Pictures gave the entire story an overhaul when the company acquired the film rights from author Gary K. Wolf. The studios hoped to make a family-friendly blockbuster in order to rejuvenate their flagging animation department, and saw Who Censored Roger Rabbit? as a means to that end.
13. I Am Legend
In 2007, Will Smith starred in author Richard Matheson's I Am Legend as Dr. Robert Neville, the sole survivor of a worldwide plague that turns humans into infected, vampire-like creatures.
The book ends with Dr. Neville, who spends his days slaying the infected to protect himself, learning that he's considered a monster to the creatures who are now the dominant race on the planet. He's imprisoned and later executed for his crimes. In the movie, however, Neville solidifies his hero status by handing off a cure for the virus ravaging the planet to a healthy woman and boy. An alternate ending that showed more interaction between Neville and the creatures was shot, but the filmmakers opted to go with an ending in which Will Smith sacrifices himself for the sake of the human race.
14. First Blood
The first Rambo movie is based on the novel First Blood by author David Morrell. The book and the movie both tell the story of a troubled Vietnam War vet, but the book ends with his death after a violent showdown with Chief Teasle. In the movie, Rambo and Teasle survive, and Rambo turns himself into the authorities.
The reason for the change: Once again, early test audiences didn't approve of the original ending, and wanted to see Rambo live to fight another day.
15. The Body Snatchers
The 1956 black-and-white classic, based on Jack Finney's The Body Snatchers, ends with protagonist Miles ranting and raving ("You're next!") along a busy highway of pod people and non-believers. But in the book, the titular body snatchers flee Earth after Miles discovers where their pods are grown and begins to set them on fire.
Though director Don Siegel and screenwriter Daniel Mainwaring were happy with their unsettling ending, the movie studio demanded a more hopeful outcome. To keep the bosses happy, the filmmakers added in a brief epilogue, during which the audience learns that local police had alerted national authorities to the presence of the space invaders. Sniffed Siegel, "The film was nearly ruined by those in charge at Allied Artists, who added a preface and an ending that I don't like."