The relationship between the Hitchhiker's book and the movie is that of Lunar: Silver Star Story Complete and Lunar: Legend.
I've played about 1/2 way through Legend, and it's not bad. The translation is clean, the story is present, and the characters present themselves well. It's not a bad translation or a bad game at all.
However, and this is a big however, playing through L:SSSC is a completely different experience, even though both games tell the same story. There is an air of polished-ness and something intangible about SSSC that pulls you in better, and deeper. It becomes less the telling of a story and more the experiencing of it, if that makes any sense.
Personally, given my experience with the book, I have to say this is most likely the fact that Douglas didn't finish the script himself. I'm not going to put blame on the other guy, because he had exceptionally large shoes to fill. It doesn't change the fact, however, that he had way too much wiggle room at his toes.