What are the major differences between the film and the novel Fahrenheit 451?
There are several major differences between the book Fahrenheit 451 by Ray Bradbury and the movie Fahrenheit 451 directed by François Truffaut including the character of Clarisse, the book that sparks Montag's imagination, and the way the future is depicted in the setting.
Clarisse is a 16-year old girl in the novel and she dies in a hit-and-run accident. In the movie, she's 20, she's a teacher, she survives for the length of the film, and she ends up in a romantic relationship with Montag. Truffaut uses Clarisse more as a companion to Montag instead of a person whose company he briefly enjoys and whose death haunts him. In the movie, Clarisse also connects him to the book people, a group who each memorize a different book to preserve it.
In the movie, Montag gets the book Tales of Mystery and Imagination by Edgar Allan Poe. In the novel, he gets Ecclesiastes, a book of The Bible.
The way the future is depicted in the novel is very futuristic with a lot of science fiction elements. According to a movie review in The Dissolve:
The director isn’t terribly interested in science fiction. The futuristic elements of Bradbury’s book, like a mechanical hound that torments Montag, have either been eliminated or toned down considerably. Firemen can go up their poles as swiftly as they go down it, and people travel via a sleek monorail system, but the high tech is kept to a minimum. The wall-sized TVs that festooned Bradbury’s living rooms are rendered here to dimensions uncannily akin to today’s average HDTV sets, which are large, but not all-consuming and immersive in the same way.
The futuristic world in the film has been scaled down to a place that's more relatable to a viewer perhaps than the world in the novel.
There are three major differences between the novel and the film of Fahrenheit 451. First, the film is set in a contemporary near-future, without many of the dramatic science-fiction elements seen in the novel; the Mechanical Hound and the wall-sized television screens are absent, as are the nihilistic children and the jet-powered cars.
Second, the war, which is a major plot point in the novel, is barely mentioned; while in the book it was a source of concern and provides the climax, in the film, it only appears in a throwaway line in the background.
Finally, the character of Clarisse is changed dramatically. In the book she is a very young girl; her influence on Montag is through her ideas, and is killed halfway through.
"There was a girl next door," he said, slowly. "She's gone now, I think, dead. I can't even remember her face. But she was different. How? How did she happen?"
Beatty smiled. "Here or there, that's bound to occur. Clarisse McClellan? We've a record on her family.
You ask Why to a lot of things and you wind up very unhappy indeed, if you keep at it. The poor girl's better off dead."
(Bradbury, Fahrenheit 451, Google Books)
In the film, Clarisse is a teacher, trying to affect the minds of her students through unconventional teaching methods, and becomes a love interest for Montag. Instead of dying, she escapes the city with Montag and they start a new life together. This goes against the novel's themes of entropy and humanity striving against the constant grind of time, but Bradbury himself enjoyed the changed premise.