Psycho's Most Shocking Twist Came From A Hatred Of The Original Book
So much of the debate in fandom today is about fealty to the source material. This shows up in comic book movie discourse, of course, and even when a franchise (like "Star Wars") is racking up sequels. There's always the hardcore fans that demand purity, that whatever movie or TV adaptation or sequel be as true to the source material as possible or their precious IP will be ruined forever (or at least until some years pass and suddenly they're angry about the next thing).
It should be noted that the most vocal fans don't really know what they want nor do they have any understanding of what needs to go into translating something from one format to another. There are a few examples out there of adaptations that worked way better than the source material. "Jaws" is one of them, "The Godfather" is another. Both are way better than their novel counterparts and if either of those books had been faithfully translated to the screen we would have been robbed of two stone-cold all-timer classics.
Turns out screenwriter Joseph Stefano didn't have that problem when he adapted Robert Bloch's "Psycho." His opening gambit when interviewing for the screenwriter position on the film for director Alfred Hitchcock was to tell him the book would just straight up make a bad movie if they told the story in the same way. His solution to the adaptation intrigued Hitchcock and gave us one of the most shocking moments in all of genre storytelling.
Stefano and Hitchcock conspire to steal the film's central character
Bloch's novel focuses primarily on the character of Norman Bates, the "Aw, shucks" motel clerk who has something of a mother complex, and Stefano was sure that audiences wouldn't want him to be the focus for the whole movie. So, what if instead, they didn't introduce Norman until 20 minutes into the movie? As he told Cinefantastique:
"The idea suddenly struck me to begin with Marion, suggesting that the movie would be about a girl who steals $40,000. Audiences would be sucked into a character who did something wrong but was really a good person — they would feel as if they, not Marion, had stolen the $40,000. When she dies, the audience would be the victim! And that's just how it worked. With so much early emphasis on Marion, no one dreams she'll get killed. When it happens, people are blown away. It's like Hitch and I were saying we've stolen your central character!"
This take intrigued Hitchcock and Stefano got the job. They both agreed in order to sell this conceit they would have to cast a star of some renown to really fool the audience, so they cast Janet Leigh, and that's how we got one of the best gotchas in movie history.
By the way, the screenwriter still had reservations about Norman Bates as a character. He didn't like him on the page, thinking he wasn't likable enough to carry the twist. Hitchcock was already ahead of him on this point, though, and told him that the problem would be solved by casting and told Bloch to picture Tony Perkins. Stefano said, "I knew then I could write the character."