If you haven’t read Divergent (which by the way i really reccomend)…I’m so sorry for the spoilers.
Divergent was interesting, but the climax was where the book really grabbed my attention, you know, the part where they forced nearly the entire population of Dauntless to kill the Abnegation. That kickstarted the series for me, and then I had to know what happened to Tris and Four. Can their relationship survive the civil war that is tearing their life apart? Can Tris actually face what she did while trying to survive? Will Four ever reconcile with his father? Then I also had these other, less character based questions like: What exactly was Erudite’s purpose in killing nearly an entire faction? What secret did Tris’ parents die to protect? What happened to the Dauntless when they realized what they had done?
Insurgent was more or less a full on kamikaze of answers for me. I got unexpected answers, answers that raised new questions, and answers to questions I never actually asked. It was the world building novel, it helped me understand how each faction worked (Tris, Four and their allies get to be around each faction). It also built upon Tris’ character, and the lengths at which she would go to protect people. Insurgent made the world more real to me. It made it conceivable. There’s a reason that each community developed, and more importantly it sets us up for the bigger question…but I’m not telling you what that is, you’ll have to figure it out for yourself. :]