I'm not sure if you're writing an essay or focusing on a certain aspect of the novel, but below are some quotes that address significant parts of the book and characterization.
1. When Perry asks Peewee why he joined the army, Peewee responds, "This is the first place I ever been in my life where I got what everybody else got" (15). Each character's reason for joining the army during war time is different, but Peewee's reason demonstrates why he is such a fighter--all his life he has had to fight to protect or keep his own.
2. When Perry and Peewee arrive in Chu Lai, they're told to keep themselves alive because "[they'll] be moving down to the Third Corps and then ship over to Hawaii from there" (33). This statement from the captain is just one example of how no one really has any idea how long the war will last, how many deaths will occur, or the gravity of it. Perry constantly hears rumors about the end being near or fantasy-type leaves that the soldiers are entitled to--none of them are true, and Perry begins to be disillusioned.
3. When Lt. Carroll is killed, Monaco prays the prayer that Carroll prayed for other soldiers. It reads,
"Lord, let us feel pity for Lieutenant Carroll, and sorrow for ourselves, and all the angel warriors that fall. Let us fear death, but let it not live within us" (128).
The prayer is significant because it supplies the symbolic meaning of the novel's title. Lt. Carroll sees the KIA soldiers as fallen angels because they are dead (fallen) at a young age (angels). However, Perry's interpretation of the title is that they are fallen angels because they have lost their innocence during combat--almost like Satan's fall from grace--not that Perry believes that he and his friends are evil; he just recognizes that they have done things that most humans never have to do or even see.
4. Near the novel's end, when Perry and Peewee are hospitalized, waiting to return to the States for good, they talk to Lt. Gearhart over the radio who tells them that "Captain Stewart had been promoted" (304). This simple statement demonstrates that even at the novel's end, Perry and the others do not know what they're fighting for. Leaders who needlessly risk the lives of them men are promoted, while those who care the most for others are killed.