I'm not sure how Guy Fawkes became a hero and blowing up the Houses of Parliament an admirable political act, but V for Vendetta certainly seems to endorse both of them. (The latter seems especially odd in a movie made only four years after the 9/11 attacks.) I haven't read the graphic novel by Alan Moore and David Lloyd, so I can't comment on the fidelity or lack of it to the source, which is just as well. But the film bears the stamp of most adaptations from graphic novel/comic book sources: an assumption that the viewer will accept the movie's milieu on its own terms, without trying to haul in real-world plausibility. It's easier to do that if you have a cast capable of playing almost anything from Shakespeare to soap opera. So the presence of actors like Hugo Weaving, Stephen Rea, Stephen Fry, John Hurt, Tim Piggott-Smith, Rupert Graves, and Sinéad Cusack goes a long way to keeping V for Vendetta alive. I particularly liked Roger Allam as a rabble-rousing news commentator in the mold of Rush Limbaugh or Glenn Beck. I was less impressed with Natalie Portman, whose British accent came and went fitfully and who generally seemed at sea. It may be that the script by Andy and Lana Wachowski called for her character, Evey, to be off-balance through most of the film, but I failed to connect with her performance, which since she is meant to be the audience's point-of-view character is something of a fatal flaw.
