In the book, Cathy is in the attic as Chris is downstairs stealing money form his mother and her new husband.
Before all this, Cathy was going to steal from her mother but she went into the room and saw her stepfather. Cathy had kissed him! Later on Chris was stealing and overheard the mother and stepfather talking and he heard the stepfather talk about the kiss... Chris went in the attic and was yelling at her for what she had done..
Suddenly he pulls Cathy down from the window sill and Chris started to rape Cathy. This was an important part of the book and I was disappointed that this scene was not in the movie... Although it would have been very awkward!