Book vs. Movie
What are the differences?
Although the settings were set up completely different so that they cannot really be compared, I think that the novel did a lot better in grasping how dangerous and close to death that Katie and the kids were. In the novel, Katie wakes up to Jo, the angel, waking her up in her dream to tell her that Kevin is there and has set the house on fire, while in the movie, Katie sees him pouring gasoline on the house and a firework is what sets the house on fire. Also, the book better portrays how deeply that Katie feels for Josh and Kristen as she jumps out of the top floor pulling them on top of her to protect them. After Katie jumps, Kevin beats Katie until she can hardly move while in the film, Alex is easily knocked out with a crowbar which takes away from the symbol of a "safe haven" that Alex represented throughout the movie.
In the movie, Katie is different. She is much more outgoing and less paranoid about everyone else. She makes a lot of friendships that she didn't make in the novel. Especially towards the end of the novel, she shows her true identity as she stands up in front of Kevin not letting herself fall back into his trap nor fall into being vulnerable out of fear of what he could do. As she starts to spend more time with Alex and his kids, in the novel, it seems as though she only spends time with them because of Kristen and Josh, while in the movie, she is more into Alex. The movie didn't really grasp the fear and vulnerability that consumed Katie's personality as she moves to Southport.
Alex and Katie's Relationship
In the movie, Katie is extremely rude to Alex as she gives him back the bike, but her feelings for Alex become more prominent and quickly than they form in the novel. In the novel, it takes many long talks and separate occasions for her to finally admit to herself that she is interested in Alex, and because Alex was a retired soldier, he was already aware that Katie had been in a previous abusive marriage because of the way she acted and in little things that she did. Therefore, in the novel, he is understanding of everything she does while the way he finds out about her in the movie is seeing a wanted poster of her for first degree murder, so he confronts her for lying about her "false identity". In my opinion, that whole scene in the movie takes away from Alex's whole character of being understanding of everything that had happened to her because it just builds a meaningless lie that hinders Alex and Katie's relationship.
Kevin and Erin's Relationship
The movie starts with Erin running away, so it fails to show just how big of a deal it was that she was able to get as far away as she did. Viewers weren't able to see and feel just how big of a predicament that Erin was in while married to Kevin especially since things were only starting to get worse. She really thought that she would die if she stayed any longer with him. Even at the end of the novel, Kevin is projected to be scarier than he was shown in the film. In the film, he just seemed really drunk while in the novel, he had complete recollection of what he was doing. Also, the novel made it seem as though Kevin really had problems because he made many biblical references to justify the things he were doing contrary to what others were doing. Multiple times, he expressed how deeply he loved and missed Erin, and then he would contradict those feeling with the joy he would have at the thought of ripping Erin's throat out.
Erin's Escape and New Identity
The novel is very different in this aspect because Erin's escape is completely independent within the novel as for months, Erin has been plotting an escape for whenever Kevin expects it the least. She has spent countless days and months saving up enough money to get out of town, and since Kevin controls everything she does, she has saved this money from the times that he has passed out on the couch with his wallet, and she has taken a dollar or two at most so that he wouldn't notice it gone. She plans and plots everything her entire escape not involving the Feldmans at all. She leaves when Kevin goes out of town for two days on work. She goes to China Town living on the streets for days not eating, and he finds her there where she sees him before he sees her and she eventually flees to Southport. This is not how the movie is at all, however, because in the movie, she escapes directly to Southport. Again, the movie fails to grasp just how scary her escape was and how close she was to being caught and the reasons for her relentless fear of Kevin finding her.
This is completely different in the book as the novel explains the months of suffering that he rightfully endures from the loss of Erin. He does get suspended but for being drunk on the job, and he does not nationally publicize the lie that Erin is wanted for first degree murder, and he finds out about Erin's whereabouts differently as well. In the novel, he is sitting on his patio drinking watching people gathering at the Feldman household for Mr. Feldman's funeral. As he is sitting there, their other daughter comes up to him asking where Erin was because she wanted to thank her for everything she did for their family. She goes on to explain to Kevin that Mr. Feldman always looked at Erin like their first daughter, Katie, who had died of cancer. A lightbulb sparks in Kevin's head as he realizes that the hope he needs to find Erin may lie within their household. As they all leave for the funeral, he sneaks in finding Katie's social security number and tracing her to Southport. I don't think that the way the movie portrayed this scene was as effective as the novel just because it was so sporadic and it showed how desperate Kevin was for finding any information on Erin.