Hollywood has a problem with originality, or rather, a lack thereof. The major studios are businesses like any other, and businesses ultimately need to make money. And what's the best way to make money? No, not drug trafficking. It's by sticking to the same formula that's worked so well in the past (which could very well be trafficking drugs, but that's a different topic).
That's why so many big-budget movies every year are either sequels, prequels, reboots or adaptations. Movie studios don't want to risk ginormous chunks of money on original ideas that, for all they know, will flop right from the get-go (like the little-known yawner Inception). Instead, they'd much rather risk ginormous chunks of money on sequels and adaptations that, for all they know, will be critical and financial successes (The Lone Ranger, anyone?). The point is that Hollywood is pathologically risk-averse and it will almost always shun an original idea in favor of a sure thing, even if choosing the sure thing gives us movies like Twilight and Battleship, which, if memory serves, was adapted from a board game's instruction manual (like all great films).
Adaptations of novels, comic books and instruction manuals remain some of the most oft-mined sources for movie ideas. But studios aren't always faithful to the written word, frequently changing--sometimes radically--the source material in order to shoehorn the story into a studio-approved package. This is by no means always a bad thing, by the way. The films in the following list, for better or worse, were not always faithful to the books they were based on. Some were good, some were bad, a few were great, but all were different in meaningful ways from the books on which they were based. Obviously, spoilers follow...
10 Jurassic Park
Jurassic Park was the massively popular 1993 Steven Spielberg film based on the massively popular 1990 Michael Crichton novel of the same name. The film broke new ground in special effects and broke box office records around the world when it was released. But the film took great liberties with the source material, especially when it came to the book's ambiguous ending. In the novel, John Hammond is killed by a pack of tiny, meat-eating dinosaurs, and it's strongly implied that Ian Malcolm dies when the Costa Rican military carpet-bombed the entire island, and the surviving characters are indefinitely detained by officials from Costa Rica and the United States. However, Spielberg rightly foresaw the demand for a sequel movie and chose to wrap things up a bit more neatly than Crichton did, thus paving the way for The Lost World in 1997.
9 Sleepy Hollow
Washington Irving's "The Legend of Sleepy Hollow" has inspired an impressive number of works in the nearly 200 years since the short story was first published. One of the better-known adaptations is Tim Burton's Sleepy Hollow, starring frequent Burton-collaborator Johnny Depp as Ichabod Crane. In the short story, Crane is a schoolteacher from Connecticut who comes to Sleepy Hollow and tries to marry the daughter of the richest man in town, only to be foiled and driven out of town by his romantic rival and town hero Brom Bones.
In classic Tim Burton fashion, his version of the Sleepy Hollow legend is decidedly darker than the original. Ichabod Crane is a New York City police officer who is dispatched to the quiet hamlet of Sleepy Hollow to investigate a series of gruesome murders. In the course of his investigation, Crane eventually frees the (quite literal) Headless Horseman from his curse and, by doing so, causes the mother of his love interest to be dragged off to Hell in the process. So, just a bit different than the original.
8 V for Vendetta
V for Vendetta was a critically acclaimed graphic novel written by British writer and all-around nerd god Alan Moore. In his version, the overarching theme of the novel is the battle between fascism, represented by the domineering Norsefire political party, and anarchy, which takes the form of V, a Guy Fawkes-masked vigilante dead set on toppling the authoritarian government. The novel was set in post-nuclear war Britain of the 1990s.
The film, however, took place in the late 2020s and was depicted as being more a battle between liberalism and neo-conservatism (likely a result of it being an American production during the politically tumultuous years following the U.S. invasion of Iraq) than Moore's intended fascism-anarchy dichotomy. It was still a great film, just one that the story's creator wasn't necessarily happy with.
7 7. The Shining
The Shining is arguably one of the greatest horror films ever made. It's probably no surprise, then, that the film is based on one of the greatest horror novels ever written. Though the film and written versions are essentially the same, they each go about telling their stories in very different ways. Stanley Kubrick, director of the film and a notoriously difficult person to work with, didn't like the novel's reliance on the supernatural to move the plot forward. Instead, he wanted to cultivate an air of suspense and psychological tension that arose not from the paranormal residents of the Overlook Hotel, but rather from the increasingly frayed psyche of the story's main character, Jack Torrance (played to perfection by Jack Nicholson, who's had a lifetime of practice playing a person with a tenuous grasp on reality). By contrast, in Stephen King's novel, the hotel itself was the supernatural entity that drives most of the story's conflict. Same destination, but very different paths travelled.
6 The Bourne Identity
The Bourne Identity was the movie that proved to the world that Matt Damon was a force of nature. In the film and its two sequels, Damon plays amnesiac and ex-CIA agent Jason Bourne, who is on a globe-trekking crusade to piece together his past and simultaneously bring down the corrupt cabal of CIA executives who masterminded the very program that ruined his life. The novel is much more focused. Bourne still suffers from amnesia and the CIA is still out to get him, but the bulk of the book focuses on Bourne's attempts to capture a dangerous terrorist and bring him to justice. Not to mention, the CIA conspiracy subplots are altogether absent from the book.
5 Fever Pitch
Red Sox fans everywhere--and fans of comedy, in general--have tried to forget that the 2005 movie Fever Pitch ever existed. Unfortunately, the film persists both in our memories and on cable television. It might come as a surprise that such a wholly inane film has any literary provenance, but the movie was actually based on the 1992 novel of the same name. The film stars an SNL-era Jimmy Fallon, before he became cool, as a Red Sox-loving manchild who falls in love with a similarly dysfunctional Drew Barrymore. Their budding romance is set against the backdrop of the 2004 Boston Red Sox championship season. The book is pretty much the same as the movie, except there's no romantic subplot, it takes place in England, not Boston, and the sport in question is soccer, not baseball. See? The same.
4 The Hunchback of Notre Dame
Disney is no stranger to turning really dark stories into run-of-the-mill family-friendly fare. But Victor Hugo's The Hunchback of Notre Dame is on a whole other planet of darkness. In a nutshell, Quasimodo is a deformed hunchback who falls in love with a beautiful Gypsy girl. Quasi's adopted father wants to be with the gypsy girl, too. She rebuffs his advances and for her efforts, he has her killed. Quasimodo murders his father by pushing him off the roof of Notre Dame and subsequently starves himself to death cradling the corpse of the woman who barely knew he existed. Heavy stuff.
So, for their version of this tragedy, the good folks at Disney had to lighten things up a bit, including having the gypsy girl live happily ever after (albeit, still not with Quasimodo), and the hunchback finally being embraced and loved by the townsfolk. Quasi's father still dies, though. But he totally had it coming.
3 The Great Gatsby
At the time of its publication, F. Scott Fitzgerald's The Great Gatsby was considered unfilmable. This sentiment was largely borne out of the book's descriptions of incredibly lavish parties and Fitzgerald's highly nuanced style of writing. It just wouldn't translate well to film, critics thought. That didn't stop people from trying, however. The 2013 version starring Leonardo DiCaprio was by far the most successful Gatsby adaptation to date, but critics were still left unsatisfied. It wasn't so much that the Baz Luhrmann-directed film changed much of the source material (though, to be sure, it did); instead, it was the film's reliance on spectacle and over-the-top set designs that did it in. Fitzgerald's Gatsby is a cautionary tale about the gossamer nature of the American dream; Lurhmann's Gatsby is a two and a half hour music video starring beautiful rich people.
2 2. Blade Runner
Both the 1982 film Blade Runner and the novel it's based on, Philip K. Dick's Do Androids Dream of Electric Sheep?, are science-fiction classics in their own right. But while the movie deals mostly with ideas of what it means to be human and the nature of identity, the book has a more environmental slant to it, questioning mankind's destructive habits and the effects they have on the planet as a whole. In Androids, the replicants, called Androids, lack empathy and serve mostly to move the plot along. In Blade Runner, the replicants are very human-like with emotions and feelings, a fact that lends a certain moral ambiguity to the story when they're inevitably "retired."
1 The Little Mermaid
It's not really surprising that another Disney movie makes the list. Because most Disney movies are fairy tales, and most fairy tales were originally written to scare the crap out of children, the company has had to significantly change many of their stories. The Little Mermaid is no exception. Originally a Hans Christian Andersen story, "The Little Mermaid" fairly tale hits most of the same notes as the popular Disney movie, only with more tragedy and excruciating pain. When the mermaid (who remains nameless in the fairy tale) drinks the Sea Witch's potion, it feels like a great sword is slowly being driven through her body; every time she takes a step on her new legs, it feels like she's walking on sharp knives; and if she's unable to make the prince fall in love with her (which, of course, doesn't happen because he falls in love with and marries someone else), she will die soulless and brokenhearted and she will dissolve into sea foam.
So, to recap: in the Disney version, the little mermaid and the prince fall in love, break the witch's curse, and live happily ever after; in the fairy tale version, the prince marries someone else, the little mermaid loses her chance at an eternal soul, and she dissolves into nothingness. On that horrifying note, let's just call it a day.