The author herself, Veronica Roth, was present! When we asked her if there was anything in the movie that she was surprised at when it was brought to life in the movie, she didn't even hesitate for a second:
"The water tank! Tris faces this fear where she's drowning in a water tank and I had kind of forgotten about that part... then I saw the actual water tank filled with actual water and Shailene actually swimming in it and I was like 'Oh. My. God.'"