The differences between the Fault in Our Stars movie and book are very minimal considering the vast differences that usually occur in a book-to-movie adaptation. While the key points and development of the story remain unchanged in the movie, there are a few smaller details that were altered, either for stylistic reasons or simply because the movie version could not encompass as much detail as the novel.

For instance, when Gus and Hazel are in Amsterdam, their romantic dinner takes place outside in the novel, but in the movie they have their dinner inside instead. Also, during this romantic dinner in the movie, Gus proclaims his love for Hazel, but in the novel, he proclaims his love on the plane ride to Amsterdam. The reasons behind these decisions could potentially be explained by the film's director, Josh Boone, as I am sure he had a creative vision that he wanted to carry out within the movie.

A few other minor differences are that Hazel has a friend named Kaitlyn in the book, but Kaitlyn does not appear in the movie. Also, Hazel goes to visit Isaac in the hospital following his surgery in the novel, but this interaction is not shown in the movie. Gus's older sisters and their husbands and children play a key role in the novel, but they are scarcely mentioned in the movie.

Another difference between the two is that Hazel sells her beloved, though neglected, swing set in the novel, but she does not part ways with it in the movie.

These details, though present and powerful within the novel, are not included in the movie. This could be purely based off the director's vision or lack of time to carry out the story properly while still including each small detail from the novel. Either way, the movie version is very similar to the book itself in that it contains all the essential plot points and many of the beloved quotes from the novel.

How does the movie "The Fault in Our Stars" differ from the book?

The above answer covers the major differences so well. One small difference is the tone of the movie versus that of the novel and use of certain film aspects. There is a certain schmaltzy tone to this movie as it progresses that is not in the book.

As with all movies, the narrative is not as detailed as the book and it does not reveal the inner thoughts of characters with the depth that a novel can do. Much of the black humor is lost, as well, since there is not the opportunity for the reader to digest the narrator's words and, suddenly, laugh with the attendant realization of the darkness attached to the sarcastic humor. Certainly, one misses the editorial comments of Hazel, such as this one in which she contends that there is much competition among cancer victims. They want to beat cancer, but they also want to outdo others:

(...Like, I realize that this is irrational, but when they tell you that you have, say, a 20 percent chance of living five years, the math kicks in and you figure that's one in five . . . so you look around and think, as any healthy person would: I gotta outlast four of these bastards.)

The doodling text messages are really silly, and the use of obscene language unnecessary as are other scenes designed for prurient interests.  The scene at Anne Frank's hideaway is rather schmaltzy/maudlin, too.

While author John Green was regularly on the set of the film to ensure that it stayed true to his celebrated book, there were a few key differences. The following are five of the most noticeable ones (WARNING: there are spoilers ahead).

1. How Hazel and Augustus meet
In the book Hazel is in the room in which the Support Group takes place (“the heart of Jesus”) and is pouring herself a drink when she notices that a seated boy is watching her. He keeps staring at her as she takes her place next to her close friend Isaac and waits for group leader Patrick to begin the proceedings. In the movie, their meet-cute is far more movie-like. As Hazel is about to take the stairs to the Support Group, she quite literally bumps into Augustus, and they warmly smile at each other as their bodies momentarily graze.

“I grabbed a cookie and poured some lemonade into a Dixie cup and then turned around. A boy was staring at me. I was quite sure I’d never seen him before. Long and leanly muscular, he dwarfed the molded plastic elementary school chair he was sitting in.”

2. Isaac
In the book, Isaac is a close friend of Hazel’s from Support Group. Physically he is described as having blond hair and wears thick glasses until his eye surgery later in the book. Immediately following his surgery, she visits him in the hospital and they maintain a close connection throughout the book. In the movie it is implied that he is Augustus’ friend and not very close with Hazel (although she does impart advice to him after his breakup with former girlfriend Monica and maintains the same talk with him following Augustus’ funeral). Physically he is played by average build, short brown-haired Nat Wolff, who portrays the character as a McLovin’-esque goofball.

“The only redeeming facet of Support Group was this kid named Isaac, a long-faced, skinny guy with straight blond hair swept over one eye.”

“Isaac and I communicated almost exclusively through sighs.”

3. Hazel’s life outside of Augustus
In the book, Hazel has a close friendship with a girl named Kaitlyn who she confides in about Augustus. Hazel is also described as a vegetarian who regularly attends college classes such as American Literature, Poetry and Algebra. In the movie, her Mother appears to be her only confidante, and her classes are glossed over, only briefly mentioned in her conversation in the beginning of the film with Augustus in his swerving car.

“My class was American Literature, a lecture about Frederick Douglass in a mostly empty auditorium, and it was incredibly difficult to stay awake. Forty minutes into the ninety-minute class, Kaitlyn texted me back.” 

4. Hazel and Augustus’ respective families
In the book, Hazel has a very tight knit relationship with both of her parents as well as Augustus’ parents. Augustus has a close friendship with Hazel’s parents, as well as his own. In addition, he has a number of sisters who are mentioned in a few portions of the book. In the movie, however, while Hazel’s father is shown to be a warm caregiver, he seems to be on the outskirts of the key events. When he speaks to Hazel after Augustus’ funeral and knowingly states, “it was sure a privilege to love him, huh?”, the line doesn’t resonate as much as it should as he’s shown barely speaking to Augustus in the movie. Sadly Augustus’ parents barely have any screen time as well and it is unclear when Hazel would have bonded with them.

5. The Ending
In the book, Hazel receives an email from Peter Van Houten’s assistant Lidewij with the correspondences the dying Augustus had with Van Houten. Within the email’s attachments are Augustus’ heartfelt pleas for Van Houten to polish and edit his eulogy for Hazel. In the movie, Van Houten hands Hazel the piece of paper with Augustus’ pleas to him and her voiceover of this letter closes the film.

“Lidewij finally wrote back just after six P.M. while I was on the couch watching both TV and videos on my laptop. I saw immediately there were four attachments to the email and I wanted to open them first, but I resisted temptation and read the email.” 

“Okay, maybe I’m not such a shitty writer, But I can’t pull my ideas together, Van Houten. My thoughts are stars I can’t fathom into constellations.”

Despite the liberties the screenwriter took with modifying the cherished book, The Fault in our Stars movie adaptation will resonate with fans of the book as well as unfamiliar to the source material viewers. The movie ensures that the timeless story of Hazel and Augustus will live on in the hearts and minds of viewers of all ages.