Here are a few differences between the movie and book:

Movie: Travis Jordan's wife is murdered by Brandon Nichols. Book: she dies of cancer and her death has nothing to do with Nichols.

also he doesn't have a dog in the book, but he does have a sister who's only around briefly. He also doesn't lose his faith in the book. He still believes in God, but lost the will to participate in ministry.

Movie: Morgan Elliot is a veterinarian Book: she is a reverend.

Also it is not revealed until much later that she is related to Michael, who just appears as a mysterious prophet/disciple of Brandon at first.

Movie: Sheriff Brett Henchle has brain cancer or something similar that is "healed" by Brandon Book: He has an old shrapnel wound in his leg that is "healed" by Brandon

They spliced 2 characters from the book together for movie Henchle. He was not married to Dee. Another guy, who did not believe in God was married to her. She's the one that goes on a rampage with a pistol.

Movie: Brandon Nichols is originally from Antioch. Book: he is from Texas.

Movie: Brandon Nichols kills his father, who had nailed him by his wrists to a fence. Book: Brandon Nichols beats his father severely, to the point he is in a wheel chair, but he is still alive.

Movie: Brandon nichols must sacrifice someone every few years in order to stay alive or have the power he has. I am not entirely sure as its been a while since I watched the movie. This is ultimately why he killed Travis Jordan's wife. Book: Brandon does not have to sacrifice anyone to get his power.

Movie: When it seems Brandon is "healing" people, he is actually causing them to become demon posessed Book: People who are "healed" are not posessed, but the "healings" either are temporary or have a negative effect on their lives.

Also Kyle is alot more involved in the book. He is actually Travis's successor at the Pentecostal church. Kyle is green and has regular conversations/meetings with Travis in attempts to get him back in church. Kyle and Travis investigate Brandon's past together. Morgan doesn't get involved until much later.

COMPLETELY OMITTED FROM THE MOVIE: *All the flashbacks, 2/3s of the book worth, pertaining to how Travis met his wife, entered into ministry, his frustrations with the churches he was involved with, and eventual departure from the church. *All the riots near the end including the one in the Roman Catholic Church, during which the crying crucifix you see in the movie gets destroyed.

Characters omitted: Dee's husband (not the sherriff), dude that through "being healed" gains the ability to repair any electronic device just by touching it, owner of the hotel that expands into a trailer park for all the tourists that come to see the "signs" and Brandon. (He gets healed too.) The guy that worked for Mrs. Macon before Brandon showed up. Fortunteller/Psychic lady.

Movie: The three mysterious guys that keep popping up everywhere, demons I guess? Book: One is a mysterious "hitchhiker", who the sherriff keeps hunting for throughout the book, one is a strange "spirit" that the fortuneteller/psychic lady converses to for "prophecies". He turns on her eventually. I forget who the 3rd guy was, but you don't learn who they really are until near the end.

MAJOR SPOILER

Movie: only Brandon dies. Book: Brandon, the sherriff, the hardware store owner in the wheelchair, elecronics repair guy, fortuneteller/psychic lady and several others all die. Also big explosions & random deaths by riots--which includes Dee running around firing the pistol haphazzardly.

END MAJOR SPOILER

One good thing about the film, probably the most accuract (though not 100%) portrayal of an exorcism I've seen in a movie. None of that over-complicated Roman Catholic mumbo jumbo.

On more thing I would like to add, especially regarding the flashbacks about Travis's past....

There is alot of stuff in this book about different Christian denominations, worship practices, and even some stuff about "megachurches" and church leadership. Frank Peretti, the author, is in fact Pentacostal, and he takes a subtle jab or two at his own denomination in regards to a couple issues (speaking in tongues & faith healing most noteably.) I have to say, even with the amount of studies I have had regarding theology and different Christian denominations, I had to do a little additional research to adequately follow some of the things Peretti discusses in the book. I can understand this content being omitted from the film, as someone not that well versed in it could very easily offend alot of people by misrepresenting it on film. To adequately portray this stuff on film would be extremely difficult.

Lastly, I'd just like to mention that Peretti has in fact been wanting to get into film directing for quite a while. Also he has been trying to get This Present Darkness made into a movie for many years. In fact, movie people started asking Peretti about adapting it long before Peretti's existing 3 films (Visitation, Hangman's Curse, & House) were greenlit. It just hasn't come to fruition yet. If you watch the behind the scenes featurette on the Hangman's Curse DVD, Peretti mentions the matter briefly.