It is not dark enough in Dipper's room, and that leaves too much room for imagination. It's also not light enough either; the only light is from the hallway and it's too distant for his comfort. The light and the darkness work together to create shadows in his room, and his paranoia does the rest.
"Probably shouldn't have watched that movie," he says to himself, hoping nothing is there. He's just imagining things, right? That tall slender shadow is just his action figure, and no, his toys do not have a mind of their own. They can't just walk right up to his bed and knife him. That's crazy.
He considers throwing the covers over his head but isn't sure if that would be worse, to be unaware of his surroundings. His shoulders seize up, lightning running up his spine as he hears footsteps approach him. He peers over the covers and almost screams. Almost. Mabel knocks the wind out of him when she pounces. Her crooked smile eases his nerves and he laughs awkwardly. "It's just you."
"Not just Mabel." She narrows her eyes, pretending to be serious. She pulls out one of her stuffed animals, a rabbit with soft frayed ears. "Hoppity, too." She impulsively rubs Hoppity's ears against her cheek, and then, invites herself under the covers next to him. "I'm scared," she explains, and Dipper forgets his own fear, wrapping an arm around her protectively.
The looming shadows are ignored as they whisper under the covers. Their only worry is that their parents will find them still awake past ten. They talk about everything but the movie. Eventually, Mabel's words slur and the conversation drifts, a sign that she is falling asleep, Although she is only half-awake, he feels comfort in her presence.
He tries not to think about the movie, of objects moving on their own or eyes rolled in the back of that little girl's head, crawling backwards up the ceiling. He remembers the paralyzing fear he felt watching those scenes while Mabel laughed in the background. He still couldn't figure out what was so funny about it-
He reddens. "You weren't scared at all."
"Nope."