The twist in the movie lies in the heart of the plot. In the novel the Doctor Montague sets up an experiment to see if Hill House is actually haunted. He plans to do this by bringing 3 people into the house to live, who have previously had some supernatural experiences. His four consist of himself, Eleanor, Theodore, and the heir to the estate Luke. Later in the novel both his wife and her assiatant visit him. The movie has the Doctor conducting a secret experiment about the primal reaction of fear he has the cast believing that he is researching insomnia. The three insomniacs remain constant except that the Doctor brings two assistants with him who leave after the first night after the House almost takes on of their eyes. The Doctors wife never comes in the Movie, and actually there is no mention of a wife at all.
The detail of character development is lacking greatly in the movie, whereas the novel conveyed the characters in great detail.
The casting was maybe the single outstanding aspect of the film. With all characters fitting their roles well, including an especially excellent performance coming from Catherine Zeta Jones as Theodore. Catherine Zeta Jones not only acted tremendously but displayed extreme grace in the visual arena also. Although she was a bit classier then Theo of the novel she was sure to be a crowd pleaser. And the movie Nell really came to life although she does not crash into a tree, but rather stays with "the children", and takes her own life.