I was surprised at some major differences from what actually ended up on screen.
I will list the differences. This will contain spoilers for those who haven’t seen the movie. (this happens to be my favorite film, I just love it in all it’s cheesy glory.

1) John (hero of the film) has two daughters instead of one as in screen version. The other is a younger girl of 6 named Abigail. They also have a dog, Rusty. Susan was 14. She appears older in the film.
2) John and Melanie were in a romantic relationship but it ended when he doesn’t believe her story of being attacked in the house.
3) Eliot West doesn’t know John already as in the film, they meet later. Eliot and Nancy become an item, even hitting the sheets.

4) Sanders being attacked by flies isn’t shown, just John finding him dying on the landing.
5) There’s no mention that John wants to write a novel.
6) No elevator scene.
7) Melanie is attacked by flies in the basement, not cold air (similar to the Sanders scene in the film).
8) Mr. Caswell returns to the house to collect his equipment and afterward, he is the one who suffers the spontaneous combustion death in his car that Melanie does in the film.
9) There’s no scene of Melanie seeing a demonic face in Sander’s photos, though she does see the blurs in his photos. After a final argument with John about her fly incident, they break up and she never appears in the script again.
10) Abigail sees the ghosts of Sanders and Caswell after they die and says they live in Daddy’s house now.
11) The housekeeper is named Sadie and has an incident with the vacuum cleaner turning on by itself, unplugging itself and John’s room being messed up after she had cleaned it.
12) No scene of John’s bathroom becoming filled with steam and walls closing in on him.
13) Emma Caswell shows up at the house after Susan dies and tells Nancy it’s John’s fault that her husband and Susan died. Nancy lets her in but while Emma is there she sees items floating in the air and runs out of the house in fear.
14) The big one is that John goes down in the basement on the night of Eliot’s investigation and is locked in it and then he catches on fire like Caswell (Melanie in the film) and stalks the people in the house like a zombie. He then dies on the lawn after everyone escapes.
15) Eliot doesn’t die and seems at the end him and Nancy might make a family with her remaining daughter, Abigail (and Eliot had lost a wife and daughter to an accident).
16) Lisa has dark hair (I thought LL should have played Lisa and MR played Susan based on the casting for Nancy and John).
17) There’s also sets not used in the film. Susan’s and Lisa’s school locker room where the comment Lisa makes about sex and ghosts is said. And there is a house where Abigail stays for a party the night of the investigation. Eliot’s apartment is a set. Scenes of John and Eliot in Eliot’s van.
A character not shown in the film is a friend who watches Abigail (Of course, neither are Abigail and her dog in the film either). Also a character who appeared to be a homeless man and walks in on Susan and Nancy when they snuck in the house and in the script, forgot to lock it. But it seems John might have hired the guy to scare them. If he didn’t, he wasn’t bothered by the man and even paid him. I was a little confused with the scene.