How is a novelization written? Just off a copy of the script? (do you have to go and view the script somewhere rather than take it away?)
How is it decided what goes in and what is cut?
Novelizers generally work from the shooting script, yes: they're sent a photocopy of it. (Or, more accurately, the publisher is sent a photocopy of it, which the editor then gives to the writer.)
Sadly, in this leak-on-the-Internet-conscious age, a lot of studios are refusing to release scripts. I know for a fact that the novelization for
Fantastic Four: Rise of the Silver Surfer had to be done by someone living in L.A. who could go into the studio and work off the script while being supervised by the producers. That is a ludicrous and insane way to do it, and will likely result in a weaker novelization (and it's even more ridiculous when you consider that
FF:RotSS was based on a 40-year-old comic book story), but some studios are nuts.
Generally nothing gets cut, because you can't afford to. What you have to do is add things: a movie doesn't really have a novel's worth of story in it. Sometimes studios are really cooperative about it; sometimes not so much. Konstantin Films was very encouraging when I did my
Resident Evil novelizations, particularly the one for
Extinction, where I was encouraged to fill in the gaps between the second and third films, and also to create a subplot for Jill Valentine.
OTOH, other studios won't let the novelizer add anything of consequence, which often results in a thin, boring novelization.
There's also the fact that book production takes much longer than movie production. Sometimes the novelizer only has an early draft of the script to work from. Sometimes the studio will be considerate and send revisions to the script along (Revolution Studios did this when I novelized
Darkness Falls); sometimes, you're working off whatever version of the script happened to be current when the contract was signed (which is why the ending of my novelization of
Resident Evil: Apocalypse differs so wildly from what was in the final cut of the movie).