The past event you describe—witnessing the murder—is an interesting one to consider. It’s an action that involves heightened emotions. Such scenes come with risks. You don’t want it to overshadow other aspects of the story. At the same time, you don’t want to dodge a scene simply because it’s difficult to write. These concerns can complicate your thoughts as you decide whether it’s necessary to dramatize it.
So, let’s take a step back and consider the purpose of a scene. A flashback, after all, is simply a scene from the character’s past. In general, fleshed out scenes signal to the reader that an event is important. It situates the reader in the moment with the character, letting the reader experience it as the character does. Scenes offer depth, too, allowing the writer to coax out nuance, create tone and explore subtleties. When considering a flashback, ask yourself the same question you ask when you’re thinking about scenes in the fictive present: Does the reader need to see how this moment unfolded or does the reader simply need to know that it happened?
If bringing the moment to life will advance the action of the story, then a flashback may be appropriate. In Sue Miller’s novel While I Was Gone, Jo Becker’s serene and rewarding life is interrupted when Eli, a former housemate, reenters her life. This experience brings back an idyllic summer that ended in tragedy when Jo arrived home to find her best friend dead on the living room floor. The story is about Jo’s reunion with Eli, but the past plays such an instrumental role in what she discovers about herself—and Eli—that Miller puts significant moments from that summer in flashback.
Other novels don’t need quite that same treatment. Cormac McCarthy’s The Road follows a father and son in post apocalyptic America. The landscape is ravaged; ash is carried by the wind. Yet, the apocalyptic event—whatever caused the devastation the father and son move through—is not dramatized. This places the emphasis on their journey, not the larger concerns of the country’s catastrophic downfall.
Flashbacks aren’t the writer’s only option to bring the past to life. Memories can surface in snippets that are filtered through the distance of time. Specific images can convey a lot of information about the past event and show how that event influences the character in the fictive present action of the novel.