Changes from the novel (SPOILERS) - some unfortunate
A ton of SPOILERS below.
This film is pretty average. Too bad, because it's based on the best Koontz novel I've read so far. The highlight is John C. McGinley's loony performance, and that's about it.
I've mentioned changes from the novel below - along with occasional comments whether those changes were good or bad.
First, the film has more dialogue than the book does. This is likely out of necessity, as we wouldn't know the internal thoughts of the main characters.
In the book, the Templeton family is murdered in their Napa Valley home. The killer returns to his home in southern Oregon. In the film, everything is set in Washington state (and filmed in Vancouver, by the way).
In the book, Jack Templeton (Laura's brother) makes no appearance. We learn that Vess killed Jack and his wife Nina incidentally, because they lived on the same property as Laura's parents. There is no side story about Jack being an accomplished pianist. In the movie, Vess even says he killed the Templetons mainly because of Jack. This is contrary to the novel, where it's very clear Vess did it because of Laura, which by the way is consistent with sexual sadist serial killers - they typically focus on the young attractive woman in the family.
In the book, Vess shoots the father with a gun (and silencer), but in the movie he only uses a knife.
In the book, when Chyna gets into the RV, she finds another (male) dead body, hung up in the closet. We find out later this was a hitchhiker that Vess murdered.
In the book, Vess promises Fujimoto he will shove the bullets up his ass, not down his throat.
The Piper Laurie character - "Miriam" - (and the cop she calls) didn't exist in the book. Chyna does steal the Honda to chase Vess, and hopes to flag down a cop if she sees one, but it doesn't happen. This was one of the more annoying changes from the novel. First, Miriam's call to the police makes absolutely zero sense. Aside from being inexplicably incoherent, she fails to mention she saw the dead body at the gas station!
Then, Miriam chasing Vess with her car is equally senseless. Instead of at least trying to tail him, she draws attention to herself (why?) with predictable results.
In the book, Chyna doesn't find a car registration and that whole thing was also added to the film. This was also annoying - it didn't make sense how she would have ripped it in the first place, nor how the meticulous Vess would have missed half his reg ripped and gone.
In the book, Chyna gets back into the RV by not just parking the Honda across the road, but faking that there was an accident, ramming the car repeatedly into a redwood. It happens at night; when she's in the woods, she doesn't see wolves, but rather coastal elk - and they don't touch her. Vess happens to see her enter the RV when he glances in the rear-view mirror (there's no muddy footprints, as in the film).
In the book, Vess' dogs are Dobermans. And another elk makes an appearance, not a wolf.
When Chyna escapes her chains at Vess' home, she doesn't throw herself down the staircase (which seemed pretty unbelievable), but rather kept throwing herself and the chair against the flagstones, which beats the hell out of her body.
The final confrontation in the book was more interesting, and more believable. Chyna and Ariel leave in the RV at night (no beeper has alerted Vess) and start driving away. Chyna sees a cop driving toward her and flags him down. After the cop stops she suddenly realizes it must be Vess. Vess unloads his revolver through the RV's front windows. Chyna sideswipes Vess' patrol car with the RV before he can reload, but misses Vess. Chyna reverses the RV and smashes the patrol car directly - the bumpers hook - Vess is soaked in gasoline but manages to roll out of the moving car with his shotgun. Chyna tries to accelerate past him but Vess unloads his shotgun, blowing out the RV's tires on one side - the RV flips over and crashes. Chyna and Ariel (wearing seat belts) survive and get out of the vehicle. Vess catches up and smashes Chyna to the ground with the unloaded shotgun. When it looks like he's going to kill her, she remembers the lighter in her pocket, and sets Vess on fire.
The ending in the movie had a lot of problems. There was no explanation why the dogs were no longer a problem when they returned to Vess' home. And certainly, Vess would have smelled the gas or whatever it was that Chyna poured on the floor. I can only assume they didn't end it like in the novel because it would have cost a lot more money.
Stuck in purgatory.