All of the characters in Light in August seem to be haunted in one way or another by their past. Faulkner uses flashbacks as a motif to emphasize the way the past is constantly interrupting the characters' present. For example, Chapter 6 introduces a flashback to Christmas's early childhood at the orphanage:
Memory believes before knowing remembers. Believes longer than recollects, longer than knowing even wonders. Knows remembers believes a corridor in a big long garbled cold echoing building of dark red brick sootbleakened by more chimneys than its own [...]
This flashback comes over Joe Christmas on the night he kills Joanna. In particular, the flashback centers on the traumatic experience Christmas had when he accidentally witnessed the dietician having sex. She caught him lurking behind a curtain, where he had been sneaking tastes from a tube of toothpaste that to him was like forbidden candy. At five years old, Christmas did not understand everything that was happening to him. He only knew that the dietician found him when he threw up the toothpaste. His childish gluttony led her to berate him, accuse him of spying on her, and call him a racial slur. She began treating him as an enemy. As his first encounter with sex and clandestine relationships, this moment instilled in Christmas a strong association between sex and violence, disgust, self-hatred, and self-blame. It is no wonder he has had a tumultuous relationship with Joanna. While his violence is not excusable, it begins to make sense in light of the flashback.
This difficult passage demonstrates that for Faulkner, flashbacks are more than a narrative device to reveal information to the reader. The line "memory believes before knowing remembers" gets at the difference between conscious and unconscious memory. To believe is to feel with one's whole being that something is true. Memories from early childhood especially might form the basis of beliefs people carry around with them, even if they are not consciously aware of the memories. Christmas has been carrying this memory around with him his whole life, even if he has not fully thought it through before. Eventually, Faulkner claims, "knowing remembers." When something in the present triggers a feeling or challenges a belief, a person might begin consciously remembering the event that first formed that belief. Finally, on the night he murders Joanna, Christmas "knows remembers believes" the traumatic experience that has led to his violent relationship with Joanna. He still believes everything he learned during the experience, but he starts consciously remembering where those beliefs come from.
Other characters, too, are subject to flashbacks in which their past bubbles up and takes over their present. Hightower, for instance, can hardly keep his mind on anything but flashbacks. All of the characters in the novel have their own struggles. Still, they all share a common challenge, which is to find a way forward even when the past won't stop coming back to haunt them. A few characters, such as Lena and Byron, are successful. Many characters die or lose their minds trying.