The movie is silly, intermittently funny, with a deadly dull patch in the middle. It's hard to keep up the momentum when you destroy the planet Earth in the first 20 minutes. Only after seeing the movie did we realize how slim the story is in Volume 1. Rather than stick to the novel (and release a 75-minute movie) or combine Hitchhiker's with Restuarant (and release a 150 minute movie), Douglas Adams included an original subplot to beef up the story. In the subplot, religious leader Humma Kavula [John Malkovich] blackmails Zaphod into retrieving a "point of view" gun from Magrathea. In the process of escaping the Vogons, Trillian is arrested and must be rescued from execution by the Ravenous Bugblatter Beast of Traal. This subplot is only tolerable because I know/suspect that Douglas Adams wrote it himself.
The casting is pretty spot-on: Martin Freeman is perfect as Arthur Dent. The romantic subplot between Arthur and Trillian (Zooey Deschanel) undercuts Trillian's brainy aloofness. I don't think there could be a "definitive" Ford Prefect or Zaphod Beeblebrox, but Mos Def's interpretation is worthwhile, and Sam Rockwell crosses Michael Keaton's Beetlejuice with George W. Bush to create a shag-rockin', cowboy-booted President who "doesn't have time for reading". (April 30, AMC Fenway; June 26, Somerville Theater)