I read the book a couple months back, and watched the movie today.
The book obviously wins for me.
SPOILER
The main difference was how Andrew died in the book vs in the movie, and also on some level, Andrew and his mom's psychology.
In the book, I think when Nina opened the attic she found Andrew to be dead already.
Millie basically tortures Andrew back, using his phrases on him, "I will let you out, just not yet."; and asking Andrew to pull out his tooth. Tbh I think the movie may have justified it better, with him and his mom being obsessed with his smile.
In the books, I did not understand why she asked for his tooth particularly, (did I miss it?)
In the movie, Nina opened the door and Andrew lunges at her, and Millie and Nina kinda work together, and Millie pushed him from the top of the stairs over the banister, and he dies. The whole dialogue between their fight was so weird tbh, it felt generic instead of like Andrew.
I absolutely loved Millie's secret man-eater sorta personality in both the book and movie, it was realistic in its own way. Sydney played it well.
What I loved in the books was the ingenuinty that Millie leaves Andrew to die after torturing him, instead of falling for his words and opening the door, as usually happens.
by Ok_Impression_8145