Making a movie out of Douglas Adams' book "The Hitchhiker's Guide to the Galaxy" is one of those clearly impossible tasks that only the foolish or seriously uninformed would do. The uninformed would make "Lost In Space" part two with British accents. The foolish, on the other hand, would have the ability out of sheer luck to make something worthwhile. They would have to make sure to stick closely to the plot and not do any mucking about with the jokes and for God's sake don't...
"Screw it." That is apparently what Garth Jennings, the director and obvious fool, said. He, the fool, walked into the project with both eyes wide open and took a sledgehammer to the novel. He put himself in peril of alienating the Hitchhiker's fan base and making a movie that would confuse the heck out of everyone who hadn't read the book.
The stars definitely plotted evil against Garth Jennings.
I saw the movie twice yesterday. The first time I went into it hoping to God that they didn't change too much. I came out with mixed feelings. By the time I had to go in the second time I was most anticipating the changes.
I don't know how Jennings did it, but somehow he made a movie that is just about, but not entirely, exactly unlike the novel... yet should still appeal to fans and non-fans alike. While people have speculated that the movie is too jumbled and confusing for someone not already familiar with the books, I personally saw it with five people who never read the book and they all followed it fine. And enjoyed it, recommending it to people entering the theater.
It will be the hardcore, uneducated fans who have the biggest problem with the movie. These are the fans who have read the books over and over, but don't realize the books were based on a radio script Adams write as a starving artist with the BBC. These fans won't realize that every incarnation of Hitchhiker's Guide has been different from the last. That's just how Adams did it. They also won't realize that Adams worked heavily with the script. He was the main writer of it up to his death. I would suspect that most of the major changes were from his pen.