Adults actually find this book (and film) scarier than kids do, with the unsettling feeling of the Other World just not being quite right giving way to its later decaying transformation into the Beldam's lair.
Following that we have the shot of the door to the Other World getting closer and closer to the real world before Coraline slams the door on the other world finally.
During the climax of the film, Coraline struggles to fight off the Other Mother's disembodied hand. The camera zooms back to the house, which shows the door to the otherworld banging loudly and glowing ominously...
It doesn't help that after they throw the hand and the key inside the well. If you listen closely, you can hear the Other Mother whispering No.
What happened to the three ghost children.
She spied on our lives, through the little doll's eyes. And saw that we weren't happy. So she lured us away, with treasures and treats, and games to play. Gave all that we asked, yet we still wanted more. So we let her sew the buttons. She said she loved us...but she locked us up here. And ate up our lives.
It's no better in the book:
She left us here... She stole our hearts, she stole our souls, and she took our lives away, and she left us here, and forgot about us in the dark.
The little ghost children in GENERAL (except for at the end, where they are kind of cute). Seriously, think about it - DEAD CHILDREN, talking about the horrors of the Other World in a distinguishably childlike way, looking like they just crawled right out of the Uncanny Valley, all with buttons in their eyes.
Notice the bed in the room. It's filthy, and has a sunken-in pit filled with dark fluid...
That one ghost girl's face, which is stuck in a permanent scream...
The fate of the ghost children, especially:
The Other Mother, in the book, swears on her mother's grave. Coraline was surprised that her mother was dead. The Other Mother's reply? "I put her in there myself. And when I found her trying to climb out, I put her back."
In the audiobook, the rats' 'song' is heard as a breathy, hissing chant with little semblance of a tune. It comes right the hell out of nowhere and is arguably the creepiest part of the whole story. And the book is narrated by the author so you know this is the way the song was meant. And then in a later chapter it happens again.
"You'll all get what you deserveses,"
when we rise from underneath."
The (admittedly rather obscure) musical adaptation has a short little ditty called "The Greeting Song," which makes excellent and frightening use of Last-Second Word Swap.
When the Other Wybie expresses his disapproval through a frown instead of smiling like he's supposed to, the Other Mother sews his lips into a perma-grin. The stitches are cut, but not before we see what the Joker might have looked like as a child.
When Coraline discovers the remains of the other Wybie.
"He pulled a loooong face...and mother didn't like that."
The Other Father's deterioration and death.
Sssorry...ssssooo sssorry...M-mother making meeeee....
Neil Gaiman holding the buttons over his eyes and smiling in this video.
The Other Father's song about Coraline, which eerily foreshadows the whole thing about sewing buttons onto eyes.
Makin' up a song about Coraline
She's a peach, she's a doll, she's a pal of mine
She's as cute as a button in the eyes of everyone who's ever laid their eyes on Coraline
When she comes around exploring
Mom and I will never ever make it boring
Our eyes will be on Coraline!
"Our eyes will be on Coraline" references that the doll sees all and by all, they mean all. Your adventures in the woods, your yearning for a more interesting life, your parents who can easily be replaced...
This piano plays me!
"I'll give you to the count of three. One... two... THREEEEE!".
Coraline suddenly getting grabbed by the Other Spink and Forcible monster, who are made of taffy
Thief!! GIVE IT BACK!
The whole Nothing Is Scarier approach during that scene. Coraline walking down the dark isle with the dog-bats above and then noticing a giant candy wrapper wrapped over something (and it's not candy) that's hanging from the ceiling. The whole time you're just waiting for something to pop out or wondering what the hell is inside that giant candy wrapper.
Some of the pictures in the novel - especially the picture◊ of the Other Mother with a bug in her mouth. It just doesn't look right.
The American cover of the book. The girl is evidently Coraline (or a mirror reflection of her) — her body looks mostly normal, but her neck is huge and very awkward and her face looks like it had buttons sewn onto them once, then removed, leaving big hollows where they once were.
If you look at it from a Freudian sense Coraline has a lot of No Yay subtext with the Other Mother. And then there's the fact that the Other Mother is a creepy Yandere towards Coraline.
"Mothers don't eat daughters..."
The name "Beldam" may be a reference to La Belle Dame Sans Merci (The Beautiful Woman Without Mercy), a figure from French folklore who would trick travellers into entering her world and eat their very life away from them. This may add another layer of creepiness to the story for those with a detailed knowledge of folklore, as it gives the audience a little hint that the Other Mother may not only have been doing this for centuries, but she may not even be confined to that house!
The empty flat and what became of the Other Father in the book. Slowly reverting back to a shapeless form, and even when he's basically a blob, even his eyes have fallen out, he's still forced to chase Coraline out of the cellar...
Not to mention the Other Forcible and Spink towards the end of the novella, trapped in a cocoon with their bodies melding together. Eesh.
The Other Mother's hand, when it's after Coraline. It leaves scratches on the window pane, and hurts one of Spink and Forcible's dogs. And when Coraline is making plans to deal with it...the sequence where she's walking to the well and can tell that something is following her, always staying just out of sight, is one of the scariest bits of the whole book.
Another big one happens at the very beginning of the movie: the re-sewing of the little girl doll into Coraline at the beginning of the film. The braided hair is unraveled, the clothes are removed and the button eyes are pulled off, leaving the doll as a featureless, human-shaped sack of cloth that gets refilled with sawdust and made into a button-eyed copy of Coraline herself. Even before we see the Other Mother, this scene is a reflection of how easily she replaces her playthings. The vaguely soothing music, "Mechanical Lullaby", that plays during the scene softens it a little (or makes it worse, it's up to the viewer to decide).
Anybody noticed the Beldam was humming to the song in the begging of the movie? Was she actually playing it on a recorder?
It's interesting to note that in the beginning of the movie, we see a glimpse of the outside of the window. It appears to be full of stars. Now, Wybie says at some point in the movie, that (if i remember) his grandmothers twin sister fell down the well. It was also mentioned in the movie that the well was so deep that you can see stars during the middle of the day. So it's basically hinted that the other mother was STILL alive in the well, to this very day. So maybe throwing the key down the well wasn't such a good idea after all. It also explains how the doll looks like its floating when it is set free from the house.