How is free will demonstrated throughout the book?

Kurt Vonnegut's classic sci-fi novel Slaughterhouse Five (1969) is in many ways a challenge to the idea of free will. Many of the events in the novel seem predestined, or at least have a connection to one another across time that suggests a kind of fated destiny for the characters involved.
To begin with, there's a certain narrative incoherence which challenges logical cause-effect reality, upon which free will can be seen to be nominally dependent. The first line of the novel—"All this happened, more or less"—is famous and captures its oddly non-deterministic qualities. Because it moves back and forth through time in a deeply non-linear manner, it challenges ideas of causality and free will as conventionally understood. It's a very postmodern, and moreover, post-Newtonian, story. Few novels capture the curiosities of time and quantum strangeness quite as well as Slaughterhouse Five.
The title, it should be mentioned, is the name of the building where the main character Billy Pilgrim takes shelter and survives the fire-bombing of Dresden. This is an autobiographical experience, as Vonnegut himself survived the Dresden fire-bombing and the conflict in Germany as a prisoner of war.