Whitley Strieber wrote the script right?
Did the director deviate from the script?
Why does Strieber hate the movie so much if he wrote it?
I've read Strieber's book and watched the movie, and the movie leaves out half of the content of the book, some of it quite important.
In fact, I recall the book being quite bizarre and even more disturbing than the movie. I suspect the director felt that not all of the book would sit well with audiences.
At least, that's my take on it. I don't know for sure why Strieber disliked it.