Burger said that Shailene has tendency to over-shadow her heroes because of her strong personality and for her Tris, they needed a stronger and manly Four and the moment they saw the two come together, they knew that Theo was the right choice.
Shailene was a little intimidated by Theo James and unlike other actors whom Shailene Woodley would push, it was Theo James who was stronger and pushed her. This equation between the two was perfect for the script and the chemistry between Four and Tris.
The first two movies in the series were box-office successes and with Divergent III:Allegient, they seem to be heading the same way. According to the book by Veronic Roth, Tris is going to die in the end.
This ending was not approved by the fans of the film because they didn’t want Four and Tris’s relationship to end in a tragedy. Roth and Woodley prefer the ending rather than having a happy one, but fans have filed for a petition for the film to change it.
It is for time to show whether Roth allows the ending of the films to differ from that in her novel, to appease the fans.