Each medium was made with the author
Except for the movie. They had to wait for Adams to die so he would no longer be able to stop them from mutilating it.
Some fun quotes from various points it the movie's development:
Reitman thinking that forty two is NOT a good answer, and that instead the movie needs a big finish, Douglas understands that he is trapped.
"It's the worst script I've ever read. Unfortunately, it has my name on it... whereas I did not contribute a single comma to it.... I'm appalled to think how much harm that script have done my reputation over the years."
And after Adams died:
Jay Roach has hired Karey Kirkpatrick, the guy who wrote or re-wrote the screenplays of "Chicken Run", "James and the Giant Peach" and "The Little Vampire" to rewrite douglas adams draft.