When the first trailer for the Veronica Roth sequel, “Insurgent,” dropped in December, fans of the books were upset at how many changes were made for the big screen adaptation. What drew the most vitriolic outrage was the invention of a mysterious and somehow powerful box. Now the author is finally commenting on the big switch up.

Speaking to Vulture, Roth shared how she feels about having her source material changed so drastically by the filmmakers. In short, she’s upset but understanding.

“When I first learned that ‘Insurgent’ was green-lit and that they were going to make it, it occurred to me that they were going to have to change a lot about it,” she said. “The plot of ‘Insurgent,’ more than the other books, is complicated. There are a lot of people with a lot of conflicting desires. There's a lot going on. So I figured they would have to streamline it and simplify it. I didn't know exactly how they would do that, but the box is the way they decided to.”

As the trailer notes, the movie’s villain Jeanine Matthews (Kate Winslet) believes the mysterious box holds the secret to the future that her people “deserve.” It is later revealed that Beatrice ‘Tris’ Prior (Shailene Woodley) is the ideal candidate to open the box due to her status as a high-level Divergent. Now that the movie has hit theaters, savvy fans know a bit more about the box.

[MINOR SPOILER ALERT] In the film, only someone who is 100 percent Divergent can open the box. This ends up being Tris, much to the dismay of book readers. In the source material, Roth says she didn’t want to make her main character seem like anything less than a flawed human being. As a result, she only falls into three of the five factions in the books. On screen, however, things are different. “Insurgent” sees Jeanine Matthews force Tris to undergo the simulations to open the box. These trials end up leading to some very well teased action scenes that are sure to satiate fan’s appetites for blood.

Although she found it jarring to have such a major change to her books be written without her, Roth feels that it was the right choice as it found a logistically sound way to get all of Tris’ simulations in one place, where a movie audience can easily digest them.

“Insurgent” is running in theaters now. It’s the second in a trilogy of books that will lead to “Allegiant,” which Deadline reports will follow the young adult trend and be split into two movies coming out March 2016 and March 2017.