While Catherine Hardwicke's film adaptation of Twilight is largely quite faithful to the novel, it naturally does make some departures. Most importantly, though, her version captures the spirit of Stephenie Meyer's novel that millions fell in love with.
Perhaps, since most of the changes made from book to screen aren't incredibly major, it's a topic not widely discussed. However, it can be fun to dive into some of these changes and consider just how different the film might have been without them.
The Timeline
In the books, Bella moves to Forks, Washington, and starts school in January. In the movie, her move and start date are shifted to March, in order to condense the events of the plot to two hours.
The longer timeline that the book offers gives Bella a bit more time to get used to Forks and to get to know Edward. However, it makes sense that the movie would need to shorten things up for the sake of pacing and time.
The Cullen Family Crest
In the books, the Cullens try to blend in with the humans, but mostly fail, since they're a group of super-beautiful orphans who are all dating each other.
The movie has them stand out even more, by having them wear a family crest on a piece of jewelry that screams "cult" even more than their identical features and odd behavior do. There's no mention of these matching jewelry pieces anywhere in the books.
The Diner
In the books, Bella cooks for Charlie upon finding out that he can't cook at all. In the first movie, Charlie takes Bella out to the diner often, his favorite place to eat. This may seem like a minor change, but it actually does shift the dynamic between the characters a bit. In the books, Bella informs the reader that her mother often required care instead of being a caregiver herself, leading Bella to be responsible for her age and feeling the need to look after others.
Her immediate assumption of kitchen duty in the books is an example of this. In the movie, Charlie taking her out to the diner, where the waitress informs the audience Bella used to go as a child, gives a bit of background to the father-daughter dynamic.
Waylon Forge
The character of Waylon Forge was completely invented for the film and doesn't exist at all in the books. Additionally, the whole subplot about the mystery of disappearances that leads up to the discovery of James' coven also isn't in the book at all.
It makes sense to put it in the movie, however, as it increases the tension and makes the vampires appear more dangerous. It even raises the question of whether Edward himself could be responsible for the disappearances, for those who haven't read the book.
The Reveal
Here is a major change from the book to screen. In the book, Bella tells Edward she knows he's a vampire while he's driving her home from Port Angeles. It's a much more quiet and intimate moment between the two. Later, Edward wants to scare Bella off with his sparkles and asks her to accompany him to the meadow on a Saturday. This planned excursion leads to a confession of love from each of them and takes up about two chapters of the book.
In the movie, the reveal is much more dramatized: Bella wanders off into misty woods, where Edward follows and menacingly demands that she "Say it. OUT. LOUD." He then gives her a speedy piggy-back ride up the mountain to the meadow, where he shows her his killer sparkles and they lay in the grass together.
Meeting The Cullens
Bella's first time meeting the Cullens goes a bit differently in the book. First, she shows up in an ankle-length khaki skirt and all of the Cullens, except Edward's super-buff brother Emmett and gorgeous sister Rosalie, are waiting to meet her. In the movie, this scene is livened up a bit, and thankfully the khaki skirt is omitted.
Carlisle, Esme, Emmett, and Rosalie are making Italian food, based on the sensible assumption that, since her name is Bella, she must be Italian. Later, Alice and Jasper hop off of a tree and enter the room.
Spider Monkey
"Spider Monkey" is just one of many bizarre endearments the Twilight movie has to offer that the book did not, though thankfully, the movie left out how often Bella, a 17-year-old human girl, says "Holy Crow."
Other endearments like this include Rosalie's dubbing of Emmett as her "Monkey Man" and Carlisle calling Rosalie "Nice Kitty."
The First Kiss
Catherine Hardwicke sexed up Bella and Edward's first kiss a bit for the movie. In the book, Bella and Edward return to her truck after hiking to the meadow and back, and Edward gives her a brief kiss, to which Bella responds most enthusiastically.
The kiss plays out pretty similarly in the movie, with the major exception that it takes place on Bella's bed and that she's in her underwear.
Bella's Attitude Towards Prom
In the book, Bella's clumsiness makes her terrified of dances. Alice dresses her up and Edward takes her to prom, and she protests to the point of tears.
The movie is much less dramatic on this point, instead having Bella act mildly reluctant, but overall much more relaxed about the whole thing.
Victoria At Prom
Here's a change that essentially serves to set up the next installment of the franchise, New Moon. In the book, there's no mention of Victoria being at prom, although it's possible that she was and just escaped notice.
However, since Edward can read minds, it's likely he would have been aware of her presence.