The filmmakers behind Twilight worked to create a film that was as faithful to the book as they thought possible when converting the story to another medium, with producer Greg Mooradian saying, "It's very important to distinguish that we're making a separate piece of art that obviously is going to remain very, very faithful to the book.... But at the same time, we have a separate responsibility to make the best movie you can make." In order to ensure a faithful adaptation, author Stephenie Meyer was kept very involved in the production process, having been invited to visit the set during filming and even asked to give notes on the script and on a rough cut of the film. Of this process, Meyer said, "It was a really pleasant exchange [between me and the filmmakers] from the beginning, which I think is not very typical. They were really interested in my ideas,"and, "...they kept me in the loop and with the script, they let me see it and said, 'What are your thoughts?'... They let me have input on it and I think they took 90 percent of what I said and just incorporated it right in to the script." Meyer fought for one line in particular, one of the most well-known from the book about "the lion and the lamb", to be kept verbatim in the movie: "I actually think the way Melissa [Rosenberg] wrote it sounded better for the movie...but the problem is that line is actually tattooed on peoples' bodies... But I said, 'You know, if you take that one and change it, that's a potential backlash situation.'" Meyer was even invited to create a written list of things that could not be changed for the film, such as giving the vampires fangs or killing characters who don't die in the book, that the studio agreed to follow. The consensus among critics is that the filmmakers succeeded in making a film that is very faithful to its source material, with one reviewer stating that, with a few exceptions, "Twilight the movie is unerringly faithful to the source without being hamstrung by it."
However, as is most often the case with book-to-film adaptations, differences exist between the movie and original source material. Certain scenes from the book were cut from the film, such as a biology room scene where Bella's class does blood typing. Hardwicke explains, "Well [the book is] almost 500 pages — you do have to do the sweetened condensed milk version of that.... We already have two scenes in biology: the first time they're in there and then the second time when they connect. For a film, when you condense, you don't want to keep going back to the same setting over and over. So that's not in there."The settings of certain conversations in the book were also changed to make the scenes more "visually dynamic" on-screen, such as Bella revealing that she knows Edward is a vampire in a meadow in the film, as opposed to in Edward's car in the novel. A biology field trip scene is added to the movie, in order to condense the moments of Bella's frustration at trying to explain how Edward saved her from being crushed by a van.One of the largest changes was the introduction of the villainous vampires much earlier in the film than they appear in the book, with Rosenberg explaining that, "you don't really see James and the other villains until to the last quarter of the book, which really won't work for a movie. You need that ominous tension right off the bat. We needed to see them and that impending danger from the start. And so I had to create back story for them, what they were up to, to flesh them out a bit as characters." Rosenberg also combined some of the human high school students, with Lauren Mallory and Jessica Stanley becoming the character of Jessica in the movie, and a "compilation of a couple of different human characters" becoming Eric Yorkie. About these variances from the book, Mooradian stated, "I think we did a really judicious job of distilling [the book]. Our greatest critic, Stephenie Meyer, loves the screenplay, and that tells me that we made all the right choices in terms of what to keep and what to lose. Invariably, you're going to lose bits and pieces that certain members of the audience are going to desperately want to see, but there's just a reality that we're not making 'Twilight: The Book' the movie."
