The movie business is always looking elsewhere for good material it can turn into cinematic hits. From The Godfather to the MCU, this has been a most conspicuously successful tactic both critically and commercially. And even from its earliest days, with 1922's Nosferatu for instance (an unofficial adaption of Dracula), the resulting film differs from its source; sometimes, a text is chosen and stripped for its parts in the course of being committed to celluloid.
Whether streamlining books of unwieldy lengths or tidying up real life events, that films often deviate from their sources is usually just an understandable practical necessity. There are certain cases, however, where more drastic changes have been made - and not always for obvious reasons.
Whether dubious casting, shifting settings of time and place, or even altering the genre, there are some films which depart so substantially from their sources that you could be forgiven for not recognising what they were based on in the first place.
Anticipate some spoilers in this list. Though there has been some effort to be tactful here, it isn't always possible to avoid letting a few key details slip. With that in mind, these are the films that most substantially deviate from their progenitors, whether by changing plot points or even genres.
A philosophy postgraduate and prospective writer from a small village just outside of Glasgow with an undying loyalty to Dawson's Creek and Buffy, ballasted by more apparently lofty interests in philosophy and literature (though I'd argue they're on the same level!)