J. Sheridan Le Fanu's 1872 novella Carmilla is a classic vampire story that predates the better-known Dracula by a quarter of a century. It's well worth reading - dreamy, sensual, haunting - and easily findable online. Note the key word: vampire.
For the most recent film version of Carmilla, writer-director Emily Harris kept some elements of the original story - repression, sexual awakening, lesbianism - but left it unclear whether or not Carmilla was, in fact, a vampire. It's an interesting choice, made because Harris was interested in highlighting other aspects of the story: in an interview with Sophia Stewart, Harris said she steered away from the supernatural to highlight the psychological and to "dive deeper into the motivations and behaviors that turn love and openness into hate and destruction".
Filmmakers often alter, condense or omit major elements of their source material.
Sometimes this is out of narrative necessity: long novels, for example, need filleting to work effectively as stories on screen (unless you're Peter Jackson and want to fatten up a relatively short tale like J.R.R. Tolkien's The Hobbit with unnecessary additions that do nothing but pad out the running time). Shakespeare's plays are also often subject to abbreviation: for example, Laurence Olivier's Hamlet, among other changes, entirely removed two notable supporting characters, leading one wag to dub it Rosencrantz and Guildenstern Are Missing (a nod to Tom Stoppard).
Though it's less important now, at one time changes were sometimes made for reasons of censorship: the old Hollywood Production Code forbade murderers from getting away with their crime so a death in Rebecca (1940), based on Daphne du Maurier's novel, had to be changed from murder to an accident that looked like it could have been murder. Not an inconsiderable change, but it seemed to work fine.
Though it's less important now, at one time (script) changes were sometimes made for reasons of censorship.
John Huston's 1941 adaptation of Dashiell Hammett's detective story The Maltese Falcon follows the novel's story quite faithfully but there are some alterations. For example, the film hints at some characters' homosexuality but doesn't make it explicit (another dance with the Production Code) and, interestingly, Huston omits Hammett's ending, which makes detective Sam Spade come off as considerably less sympathetic, though the sense of moral ambiguity remains.
Cinematic adaptations of Emily Bronte's Wuthering Heights tend to focus on the first half - the Catherine and Heathcliff love story - which makes sense since the story is quite long and the "Next Generation" material isn't as arresting (and if included would make for a long movie). The tone is often a bit more genteel than the novel, which tends towards melodramatics that might appear excessive on screen.
When screenwriter William Goldman adapted Stephen King's novel Misery - in which Paul, a novelist, is held captive by Annie Wilkes, an insane fan - he was adamant the film needed to retain the horror of the book's "hobbling" scene. Many male stars passed on the film: the idea of having a foot lopped off was too emasculating. Eventually, it was decided Annie would break Paul's feet with a sledgehammer, which was effectively brutal and crippling without being as gory or final as the original.
The Walt Disney film Mary Poppins not only gave the episodic book more of a structure and throughline - getting a father to reconnect emotionally with his family - but changed the tone of P.L. Travers' story, despite keeping many character and story elements. The irritable Poppins of the book was softened for the film and the story's mysterious qualities downplayed in favour of Disney brightness and song. The film was a success on its own merits but very different from Travers' creation.
Other times, though, the changes, as in Carmilla, significantly alter the meaning or thrust of the source. My Sister's Keeper swapped two main characters' fates. Stanley Kubrick's film version of A Clockwork Orange was based on Anthony Burgess's novel but used the American edition of the book, which omitted the redemptive final chapter and thus ended on a very different, darker, note.
Burgess seemed to like Kubrick's film overall but Stephen King was less positive about Kubrick's adaptation of his book The Shining in which many changes were made. There was a personal element to the book that was lost - the main character, Jack, like King at the time he wrote it, was battling alcoholism. King also disliked Jack Nicholson's portrayal of Jack, which made the character seem crazy from the start rather than someone who gradually became unhinged. He felt Kubrick's cold, detached style was unsuited to the material and preferred a later TV adaptation.
When Adaptation screenwriter Charlie Kaufmann had trouble scripting a movie based on Susan Orlean's book The Orchid Thief, he inserted his struggles into the movie, turning much of it into a surreal look into the creative process. Orlean - played by Meryl Streep in the film - wasn't happy at first but came around, saying it was true to the book's themes about life and obsession.
The old advice to writers selling their works to the movies - "Take the money and run" - still applies.