After cutting her hair for the role of Hazel Grace Lancaster for The Fault in Our Stars (2014), Shailene Woodley decided she did not want to wear a wig for Insurgent. In the book, Tris does cut her hair but not as short as seen in the movie.
The character of Caleb has a very unorthodox style of running in the movie, particularly near the start. This was a choice made by actor Ansel Elgort who stated that, "Shailene Woodley and Theo James are superheroes. Caleb is not. Caleb represents the intellectual side, the guy who isn't physical. And I wanted to make sure that was on screen. I wanted to make sure that made sense."
The character Edward, played by Ben Lamb in Divergent (2014), was supposed to be in the film, but Lamb dropped out to do Blood Orange (2016) instead. Instead of recasting the role, the filmmakers chose to create a new character to take his place, Edgar, played by Jonny Weston, which some fans initially thought would be a new love interest for Tris.
Many fans criticized the fact that Tris was able to fire a gun confidently (while in the novel is unable to hold a gun in her hands, due to PTSD), she only fires it once throughout the movie & doesn't even harm anyone with it either.
In the book, Candor headquarters is in the present-day Merchandise Mart building in downtown Chicago. While a fictional building is used in the film, some exterior shots show the actual Merchandise Mart behind Candor headquarters, and the city skyline as seen from the windows in interior Candor scenes is geographically accurate if the building was located on the present site of the Merchandise Mart.
Unlike the first film where the firearms used by the factions were mostly non-firing props in this film they use real weapons. Among them are the Chiappa Rhino revolver and the Kriss Vector carbine and SMG (albeit concealed in futuristic looking shells).
In the book version of Insurgent, Tris gets injected with an overdose of Amity peace serum after her fight with Peter. In the movie, she doesn't get any injections from them, just a lecture from Johanna.
The trivia items below may give away important plot points.
In one draft of the script, the film's final scene involved Tris shooting Jeanine in the back of the head. This ending was shot, but was deemed "too dark" for a PG-13 rating and the studio ultimately feared that audiences wouldn't respond well to the film's lead shooting an unarmed character in cold blood. Ultimately, the director decided that it would be better if Evelyn shot her instead, symbolically having one villain swapped out for another.
In the book Tris had to work with Marcus (Four's father) to reach the computer where the message was stored because Four would not let her go with him. Four found her before she could open it and got upset with her for working with his dad, only to see and display the message to everyone the end of the book and partially forgive Tris.
Due to Tobias/Four and Tris both growing up Abnegation they do not have intercourse until the sequel, Allegiant, in the books, while here it is suggested that they have it before Tris leaves for Erudite headquarters.
In the novel, Hector, Marlene and a girl named Kee are under a simulation and are about to commit suicide when Christina and Tris save two of them. But in the film Christina, Hector and Marlene are about to plunge to their death, when Tori and Tris save Christina and Hector. Additionally in then novel the scene takes place at Dauntless HQ whereas in the film it is among the Factionless.
In the Divergent novel, Tris gets two additional tattoos on her shoulders, one of the Abnigation symbol the other the Dauntless symbol. This was never in the movie however, when she greets Christina at the Candor headquarters in the Insurgent movie(2015), the Abnigation tattoo is visible on her right shoulder.
In the beginning of the movie , Peter and Tris get into a fight and Peter is left with a scratch on his face . At the end, Four has the same mark in the same place Peter(by now his has healed) had his.