I didn't want to do a cheesy novelization, where some hack comes in and kind of makes s--t up. I wanted to do something that was a legitimate novel that was inside the characters' heads and didn't have the wrong culture stuff, the wrong language stuff, all that.
Yeah, heaven forfend a writer make shit up. That might be, I dunno, fiction or something!
More seriously, novelizers have to make shit up because a movie only has a long short story's worth of actual plot in it. If you're gonna get a novel-length story in there, you have to add stuff.
Whether or not that shit is true to the film generally depends on the level of cooperation the film studio provides the publisher of the novelization. As a for-instance, the producers of Darkness Falls were hugely helpful, providing me with a ton of backstory that didn't make it into the final cut of the movie. As another, the producers of Resident Evil: Extinction encouraged me to add a ton of material -- lots and lots of "making shit up" -- to bridge the gap between Apocalypse and Extinction, and also to fill in what was happening with the Jill Valentine character. As a third, Serenity had fourteen hours of televised episodes of Firefly as additional background.
And sometimes producers don't cooperate at all and the novelizers don't have a choice to make shit up. That's not hack work, that's writing and creating. But, y'know, it's just prose, so it doesn't count. It's not real writing, not like a script is.... *rolleyes*
As ever, I am amused by the fact that a writer who adds plot and characterization to an existing story in order to make a movie into a novel is dismissed by people like Cameron as hacks, while screenwriters who (ahem) hack away at a novel's story and remove huge chunks of it in order to whittle it down to a movie's length get their own Academy Award category.
This is why we formed the IAMTW, to combat this obscene prejudice against our craft. Ignorant comments like Cameron's are a good reminder of how far we have to go.