In The Bridge of San Luis Rey, Wilder makes frequent use of flashback to illuminate his characters' lives and the novel's core themes. The novel's three middle chapters, which detail the lives of the bridge collapse's victims, each start at the beginning of one character's life and proceed until the final tragedy before the next chapter moves back in time. While these chapters are far longer and more elaborate than a typical flashback, they still interrupt the forward flow of time in order to give the reader a better understanding of each character's trajectory.
This formal choice reflects the novel's central conceit: Brother Juniper has set out to investigate each victim's past and record their stories in a book. The novel's three middle chapters represent the results of that project. (It's also important to keep in mind that they're conveyed by an omniscient narrator who has far more insight than Brother Juniper into the characters' interior lives.)
The flashbacks also help the novel explore central questions about divine power and individual human will. In writing his book, Brother Juniper is trying to find evidence of God's action in a seemingly random accident—in other words, to logically "prove" why some people die while others survive. By making the victims' fates known from the start, Brother Juniper is asking the reader to inspect the characters' life stories and judge whether or not they deserve to die.
Of course, Brother Juniper's mode of inquiry turns out to be misguided and fruitless. The story of each victim is far too complex to allow for the sweeping judgments the cleric wants to make. As the novel makes Brother Juniper's failure evident, the flashbacks start to serve another function, challenging the reader to find meaning in the characters' lives despite knowing the tragedy that awaits them. Demonstrating to the reader the passionate interpersonal relationships that define each victim's story, and showing how grief transforms survivors like Doña Clara, the Abbess, and the Perichole into better people, the novel suggests that love gives meaning to life regardless of random tragedies.
