Question: Why does Natalie look upset at the end of the Choosing Ceremony? The book says she smiled at Tris. That and why is Jeanine presiding over it? Marcus did this in the book.
Answer: For one, Natalie is upset that both Tris and Caleb have chosen different factions from her and Andrew's, which means they will have little future contact with their children. Also, Tris chose Dauntless, the faction that Natalie was born into and chose to leave. The movie audience has to be able to see how Natalie truly feels about this and a smile would give the wrong impression. Also, Jeanine presiding over the Choosing Ceremony instead of Marcus is a common book/movie type of change that often occurs in films. Here it introduces and places emphasis on Jeanine and foreshadows the importance her character will play.
Question: Before Tris went on the zip line, why didn't anybody tell her there would be obstacles in her way that could have killed her if she didn't avoid them?
Answer: There aren't really obstacles. On a zip wire, there isn't really much scope for manoeuvrability. You follow the wire.
Answer: That and they do crazy stuff like this all the time so not telling her about these obstacles is keeping in character as they leap into everything seemingly blindly.
Answer: It's typical for movies to delete or dilute various plot elements from a book source. The movie's running time is a factor, and eliminating characters, side stories, combining plot points, etc. streamlines the plot. The movie chose to keep the focus on Tris and Four.
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