Who is the narrator, can she or he read minds, and, more importantly, can we trust her or him?
First Person (Central Narrators) / Cassie and Ben
Most of the book is narrated in the first person, tag-team style, between Cassie and Ben. For most of the book, these two characters are unaware of each other's existence. (They were high school classmates in the Before Times, but they sort of lost track of one another in the ensuing apocalypse.) They're only brought together in the final chapters, when they risk their lives to rescue Sammy from the aliens' clutches.
Jumping back and forth between perspectives can be nerve-wracking, particularly when we switch from Cassie to Ben (or vice versa) during a critical moment. It builds suspense and ramps up our anxiety as we wait to see how any given situation turns out. In a way, it also mimics the experience of the characters, who are pretty much always working with incomplete information. (After Sammy is kidnapped, it's not like Cassie can just text to see if he's okay.)
Though their circumstances are very different—Cassie spends most of the book chilling in a farmhouse, while Ben's going through crazy military drills—the two teenagers have a lot in common. They're both human, and they're both dedicated to young Sammy. They're brave fighters and creative thinkers.
The real difference in their perspectives relates to what motivates them. (You know, what gets them moving in the morning.) For Cassie it's hope—hope that her brother is alive, and maybe even a flicker of hope for humanity. Ben pretty much lost all his hope when his family died. He's motivated by his desire for revenge.
Third Person (Limited Omniscient) / Evan and Sammy
We take two super-quick breaks from Cassie and Ben's first-person narration, switching gears into the third person.
In Chapter 31, the third-person narrator follows the consciousness of Evan Walker. At the time, he's a nameless alien hunter, tracking (and preparing to kill) Cassie:
He was very good at waiting. He could crouch for hours, motionless, silent, he and his rifle one body, one mind, the line fuzzy between where he ended and the weapon began. (31.2)
The switch from the first person to the third feels abrupt, and makes us hyperaware that we're reading about an alien consciousness. Someone Other. When Cassie talks about Evan, somehow he seems more human.
In Chapters 37 to 41, a new narrator follows Sammy through his arrival and processing at the military base. (This is the second and final section with third-person narration.) Through the first chapter, we're aware of his uncertainty and fear as he leaves Cassie and rides the bus to the base. As he's admitted into the camp, the narration of his processing is especially cold and sinister:
Number forty-nine has been tagged […] Number forty-nine has been mapped. (40.13)
Yikes.
Why the third person? So far as we can tell Sammy isn't an alien like Evan, which makes the author's choice a little confusing. Maybe it's because Sammy's so young, or to emphasize that he has been kidnapped by the Others—or both.