Before writing a relatively short post about the book and film versions of Shirley Jackson's We Have Always Lived in the Castle, I re-read the book and made copious notes. Spoilers are avoided in the main post but abound here.
In the book:
1. Merricat Blackwood goes shopping in town Tues and Fri, not Tues only. (Why change that?)
2. Merricat has a private "Snakes and Ladders" type game that helps her cope with town visits. ["Crossing the street (lose one turn) came next, to get to the grocery directly opposite. I always hesitated, vulnerable and exposed, on the side of the road while the traffic went by."]
3. Local men in coffee shop: Jim Donell is the town Fire Chief (we don't learn this until the end of the book) and "Dunham" is a carpenter who did some work at the Blackwood estate. Donell wasn't involved romantically with Merricat's older sister Constance. Donell doesn't put out a cigarette in Merricat's coffee.
4. A central (past) incident, referred to throughout the story, is the poisoning of the Blackwood family at the dinner table six years earlier. Constance was accused and acquitted but is still blamed by the townspeople. Merricat had a brother, Thomas, among the murdered family members. Very little is said about him. Julian, uncle of Merricat and Constance, survived and still lives with the two sisters. "Uncle Julian" had a wife, Dorothy, who also died at the table that night.
5. Julian was sickened by the arsenic and shattered by the deaths and became an invalid thereafter. The book doesn't give his age but we assume late '60s. He is depicted as much older and frailer than Crispin Glover's interpretation of him in the film. Glover captures his intermittent sharpness of mind.
6. There is more detail about Julian's original position in the family. He and Dorothy were living under the roof of his brother John (Constance's and Merricat's father, one of the poisoned family members) and he was sensitive about his dependence. This was not helped by John, who kept an eye on how much food they ate at the table.
7. The book Merricat nails to the tree is a small ledger kept by her father of monetary sums owed to him and "people, he thought, who ought to do favors for him."
8. Nosy, pushy people circling around the house calling out for Constance, after the murders, is a regular occurrence. At first the sisters assume Charles is one of these.
9. Merricat doesn't go to town a second time for sugar. She is home with Constance (but upstairs) when Constance invites cousin Charles into the house.
10. Charles' father was Arthur Blackwood, John's brother. Arthur shunned Constance and Merricat after the murders, and Merricat spent time in an orphanage while Constance was on trial. Once Arthur was dead Charles was free to visit the sisters (or so he tells Constance). Charles also tells Constance that Arthur died broke.
11. Charles is hostile and sinister towards Merricat almost immediately. On his second day in the house he says to her (pretending to talk to her cat Jonas): "I wonder if Cousin Mary knows how I get even with people who don't like me?"
12. Merricat is our point of view character so we have to guess at conversations between Charles and Constance held outside her hearing. We know he's obsessed with the Blackwood family fortune, locked in a safe in the house, and that he's attempting to persuade Constance to get outside more and "do something" about the troublesome Merricat and addled Julian. There are no overt scenes of Charles romancing Constance or speaking Italian, as in the movie. That he might have been using sex for persuasion is hinted at near the end of the book. See item 22.
13. There is no sex; it's all absent, repressed. Charles embodies the threat of maleness to the feminized household but is not a Lothario.
14. The "Merricat should never be punished" scene takes place in the estate's abandoned summerhouse. Merricat imagines the entire family sitting around the dinner table agreeing that she should never be punished. It's a fantasy but also, possibly, in some sense, a flashback. Perhaps Merricat was spoiled as a child, so badly that she never developed a conscience. Jonathan Lethem speculates about this in his afterward to the Penguin edition.
15. Charles keeps insisting that Merricat be punished for pouring water on his bed and spreading dirt and leaves around his room. He never touches her. The scene of him angrily manhandling her on the stairs, and the suggestion that the sisters are flashing back to similar parental abuse, are completely invented in the movie.
16. At the climax, after the fire is out, Donell throws the first rock through a window and the townspeople invade the house, destroying furniture and valuables. The sisters try to escape into the woods; they are surrounded and heckled but not touched. Constance keeps Julian's shawl over her face so no one can see her. The grabbing and manhandling of the sisters never happens in the book.
17. Charles keeps yelling for people to get the safe out of the house. He doesn't try to break into it, as in the movie.
18. No dramatic gunshot stops the heckling. The doctor announces Julian has died. Charles (who seems to have joined the mob) wants to know if "she" killed him. The doctor says it was a heart attack. (Not suicide by smoke inhalation as suggested in the movie.) The mob disperses because a death has occurred.
19. The catalog of destruction to the house goes on for several pages. This is shown in the movie in a series of shots of furniture, dishes, etc being smashed. The book emphasizes the sadistic, systematic thoroughness of the destruction. (Silverware removed from drawers and bent, Constance's harp thrown out a window, etc.)
20. The life of the sisters, post-fire, is very primitive. All their clothes are burned. Constance makes herself a garment out of an old suit of Julian's. She makes a dress for Merricat out of a red and white checked tablecloth. They wall themselves into the house, which becomes overgrown with vines.
21. Charles comes back to the house once. He tries to trick Constance into coming out so his friend can get a photo for the magazines. He is still obsessing about the money in the house. He stands in the driveway yelling but the sisters don't acknowledge him. (In the movie he enters the house, attacks Constance, and is killed by Merricat.)
22. After several years (?) of living as shut-ins, the sisters hear some furtive movements outside the derelict house one night. They muse that their property might have to be renamed Lover's Lane. Merricat cracks, "after Charles, no doubt." Constance replies, "The least Charles could have done was shoot himself through the head in the driveway."
23. The poisoning of the family is never openly explained. There is no subtext of child abuse or sexual abuse, as in the movie. Just hints that patriarch John was a controller and a snob and the family was hidebound in its class assumptions and conventionality. We're left to assume that spoiled child Merricat was starting to find her family disagreeable (12 is pretty old to be sent away from the table without dinner) and planned the murders in a way that spared Constance. The book is a tale of co-dependency: Constance protects Merricat and neither show remorse for the killings. The movie adds motivation of an abusive parent because all Hollywood films must have an abusive parent as a villain.