I guess Doug Adams made changes and added characters after the books were written. Wasn't there a new character in the radio show? oops that should have said TV show. When writing the screenplay, he added stuff too. I will find the link and edit this post.
So, how to be a purist?? You'd have to know what changed AFTER Doug Adams died!
I said I would edit this post...
This is a fan site......
http://homepage.ntlworld.com/mjs2000/movie.htm
Will the film be faithful to the novel?
No – because Douglas Adams did not want it to be. The novel of Hitchhiker’s Guide was very, very loosely adapted from the original radio series. The TV series was also very, very loosely adapted from the radio series, and so were the LPs, the computer game and the stage play. All these versions differ in very significant ways, often contradicting each other. The film will also contradict all these versions. The script being used is all Douglas Adams-created material – Kirkpatrick has merely tidied up the screenplay into a filmable form – but it is not a literal adaptation of the novel and features major new characters and events.
Who is in the cast?
The confirmed cast is as follows:
Arthur Dent: Martin Freeman (The Office, Hardware, Love Actually)
Ford Prefect: Mos Def (The Italian Job)
Zaphod Beeblebrox: Sam Rockwell (Confessions of a Dangerous Mind, Galaxy Quest, The Green Mile)
Trillian: Zooey Deschanel (Almost Famous, Elf)
Slartibartfast: Bill Nighy (Love Actually, Still Crazy, Shaun of the Dead)
Mr Prosser: Steve Pemberton (The League of Gentlemen)
Body of Marvin: Warwick Davis (Return of the Jedi, Leprechaun)
Questular, Vice-President of the Galaxy*: Anna Chancellor (Four Weddings and a Funeral)
Humma Kavula, ‘crazed missionary’ on the planet Viltvodle VI*: John Malkovich (Dangerous Liaisons, Shadow of the Vampire)
A Vogon Guard: Jerome Blake (The Phantom Menace)
role unknown: Simon Jones (Arthur Dent in the radio and TV versions of Hitchhiker’s Guide)
* New characters created for the movie by Douglas Adams
And I found a year-old blog of script co-writer, Karey KirkPatrick.
http://hitchhikers.movies.go.com/hitchblog/interview.htm
KirkPatrick :
"(and I don’t think we have the rights to any new material created specifically for the TV show, so for that reason, I never watched it. Do you hear me BBC? I NEVER WATCHED THE TV SERIES). "