"Don't be stupid," he told himself. "You're too old for monsters." (1.15)
Do you think there's actually an age at which you're too old for monsters? Teens might not believe any more that there are monsters in the closet or in the bed, but they still like horror movies and ghost stories and sleepover séances.
The rest of the tree gathered itself into a spine and then a torso, the thin, needle-like leaves weaving together to make a green, furry skin that moved and breathed as if there were muscles and lungs underneath. (1.36)
This is such a wonderful, creepy description. If the monster's body is made of branches and its skin is made of leaves, what might its muscles and lungs be made of?
The voice had a quality to it, a monstrous quality, wild and untamed. (1.22)
The monster says stories are wild, so it makes sense that its voice should also be wild. It's a more effective storyteller that way.
The monster smiled. It was a ghastly sight. If I must force my way in, it said, I will do so happily. (5.16)
By not telling us exactly what the monster's smile looks like, Ness is letting us imagine it for ourselves, which is often scarier than anything anyone could describe (or, in this case, draw).
Even though it walked and talked, even though it was bigger than his house and could swallow him in one bite, the monster was still, at the end of the day, just a yew tree. Conor could even see more berries growing from the branches at its elbows. (7.35)
The berries on the monster's elbows remind us of the berries Conor found on his bedroom floor after its first appearance. He'll later sleep among them when the monster makes a nest of its arms. Ness is subtly weaving a piece of characterization through the book for those of us who are paying close enough attention.
He was inside the nightmare. (21.26-27)
One of the ways people who suffer from depression describe it is by saying it changes the color of the world. Suddenly everything looks duller and darker, and it could definitely be described as being inside a nightmare. Metaphor alert!
Conor had felt what the monster was doing to Harry, felt it in his own hands. When the monster gripped Harry's shirt, Conor felt the material against his own palms. When the monster struck a blow, Conor felt the sting of it in his own fist. (24.9)
Hmm. This is an odd description. How can Conor feel what the monster feels? Well, Shmoop's theory is that fighting back against a bully who's been physically assaulting you can cause you to feel like you're outside yourself, either because of anger or because of fear. Maybe the monster isn't involved at all.
The real monster […], the real nightmare monster, formed of cloud and ash and dark flames, but with real muscle, real strength, real red eyes that glared back at him and flashing teeth that would eat his mother alive. (28.42)
Why does the nightmare monster have real muscle, even though the tree monster (as far as we know) doesn't? What does it say about the difference between monsters that one has real mammal innards (not to put too fine a point on it)?