In the begging of the movie Meg is daydreaming while in school where in the book she was in her attic at home talking to herself. During the middle of the movie Meg meet Calvin while breaking up a fist fight which did not happen in the book. Last when all the kids meet the “man with the red eyes”they lose Charlie and go see IT where Meg tell the things her father use to tell her, she does not do the when the “man with red eyes” gets into her mind.
The other comparison is relationships and characters are different from the