Casting by random factor

I've discovered the IMDB's page for the Hitchhiker's Guide To The Galaxy movie. I'd heard about Martin Freeman as Arthur and Bill Nighy as Slartibartfast but Mos Def as Ford Prefect? Sam Rockwell as Zaphod? Zooey Deschamel as Trillian? Warwick Davis as Marvin?

I need to lie down...