Terry Jones Extraterrestrial Director, Voice, Screenwriter Simon Pegg Neil Clarke Actor Kate Beckinsale Catherine Actor Sanjeev Bhaskar Ray Actor Rob Riggle Grant Actor Robert Bathurst James Cleverill Actor Suzy Eddie Izzard Headmaster Actor Joanna Lumley Fenella Actor Marianne Oldham Rosie Actor Robin Williams Dennis Voice John Cleese Extraterrestrial Voice Eric Idle Extraterrestrial Voice Michael Palin Extraterrestrial Voice Gavin Scott Screenwriter Bill Jones Producer Ben Timlett Producer Chris Chesser Executive Producer Jason Garrett Executive Producer Mike Medavoy Executive Producer David Rogers Executive Producer Mark Sandell Executive Producer Edward Simons Executive Producer Angela Fuguet Executive Producer Peter Hannan Cinematographer Julian Rodd Film Editing George Fenton Original Music Irene Lamb Casting, Casting James Acheson Production Design, Costume Design Harry Pain Art Director Keith Pain Art Director James Acheson Set Director