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.
Universal's classic 1931 Frankenstein movie has transcended cinema and is a fixture of pop culture, even if its relation to Mary Shelley's 1818 novel is loose, to say the least. Perhaps the biggest change is the most stark, as, for all its popular currency, Karloff's shambling zombie-like mute is a far cry from the eloquent, if monstrous, creation from the book.
The James Whale-helmed Frankenstein sees Henry Frankenstein (Colin Clive) and his hunchbacked assistant Fritz (Dwight Frye) rob graves to construct the Creature (Boris Karloff). There's a hell of a lot of changes to note here, starting with Dr Frankenstein being called Victor in the book. Hunchbacked Fritz is an invention for the screen, and there's none of the Euro-trotting in the film that is found in the book, whose action ranges from the Swiss Alps to the Scottish Highlands to the Arctic Circle.
As mentioned, the biggest change regards the Creature itself, who though monstrous of appearance in the book, is a highly intelligent and loquacious nemesis for Victor, with the Monster learning how to read and speak by reading Milton's Paradise Lost. Karloff's magnificent performance aside, there's no chance his stiff, cobbled together collection of corpses would be reading anything so highbrow.