I have this odd image of Colin Firth as Dent. I’ve always seen Dent as being very square, very British and oddly likeable - fits Firth to a T.
Hugh Laurie would make a great Arthur Dent. And hell, Stephen Fry for Marvin. 
Bruce Willis for Wanko the Sane.
I’m stuck on the Golgafrinchin Captain, Bill Clinton?
Zaphod should be played by Denis Leary! How about that? Loud, obnoxious, and funny.
I actually saw the TV series before I’d read the books, so I always just pictured the characters in the books as the actors who’d played them on the show.
Except for Trillian, who for some reason I always pictured as Miranda Richardson. And I totally agree with kilt-wearin’ man that Chris Barrie would be perfect as Arthur Dent.
Oh yes, Alessan - Colin Firth would make a very good Arthur Dent.
Martin Kemp for Ford Prefect?
How about a Trillian in her thirties? In which case Fay Ripley would be ideal.