This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Monsters, Inc.
Open this door! OPEN THIS DOOR!
This line from Waternoose: " I'll kidnap a thousand children before I let this company die, and I'll silence anyone who gets in my way!"
Overlapping with Paranoia Fuel, Randall uses his invisibility to absolutely chilling effect against Sulley. Since he's invisible to the audience too, we're held up in suspense once he disappears and starts scampering all over the place. An especially effective moment is when Sulley and Boo are hiding on the scare floor... and Randall materializes right beside them.
In another scene, Mike is trying to escape from Randall. He ends up in a hallway next to a portrait of Waternoose and thinks he's lost him...and then Waternoose's portrait suddenly blinks. It's Randall, blending into the colors of the portrait. Needless to say, the audience's reaction mirrors the one Mike has when he turns around to see Randall standing behind him.
The Scream Extractor, a machine that clamps over the mouth of the restrained victim and forcibly extracts the scream. It's demonstrated that it extracts the breath as well as the screaming. Had Sulley and Mike been unable to save Boo, the Scream Extractor would have left her a dead, withered husk. Or worse, if the Extractor was kept to low power, she would have been trapped in it indefinitely having the scream drained from her 24/7.
When the monsters harvest screams from little kids, a monitor displays how much screams they have collected with Celia announcing such events in a deadpan fasion. Strangely similar to the telescreen. Creepy.
On the DVD, there was an Easter Egg that showed the giraffe used to measure height on Boo's wall moving, then peeking around her door to look straight at the camera.
On the DVD, on the second disc in the "Humans Only" Menu, watch the production tour all the way through. You will then be treated to a bunch of doors, each containing an Easter Egg. One of them is Sulley scaring the simulation room kid.
The scene of Sully being strangled by an invisible Randall. It's mixed in with the comedy of Mike being completely oblivious, but hearing his breath slowly leak out as well as the dramatic music is very disturbing. Especially considering the fact that you can see his neck getting crushed.
If you don't find it funny due to his over-the-top reactions, Sully mistakenly believing his beloved Boo is in the trash disposal machine is this, as poor Sully honestly believes Boo is being horribly mangled to death.
During the final confrontation with Waternoose, Sully runs away with Boo into the Scare Simulator room, and attempts to lock the door shut with a piece of a metal pipe. Thanks to some pretty impressive lighting, Waternoose slowly bashing his way in actually makes him look quite threatening.
Though Boo "rampaging" Mike and Sulley's apartment is quite hilarious, there's a brief bit where she "corners" Sulley. Her eyes were totally focused on him and when she tosses the CD's she was holding aside, she keeps looking at him and keeps walking. She almost looks zombielike as she does this, right down to her hands reaching out.