This post links back into what I discussed how to approach your opening line.
Generally speaking, the majority (75% or more) of your story should be forward progression. Of course there are exceptions to that idea; some of the ‘greats’ have written well-received books that featured a great many flashbacks and such. That being said, your story should have some sense of forward progression. That is to say, you want it to move forward from point A to point B, not backward through character A’s life and experiences to date. Why?
You risk throwing off the flow and losing the reader.
Let’s say that you opened with a flashback, that’s quite a standard opening. That however is very difficult to pull off because yes, you’re gripping the reader, you’re starting with action, but then you snap the reader out of that. You shift from whatever traumatic event happens in the flashback to the boring everyday situation the protagonist is then in. That can then lead to the reader feeling confused and cheated. It’s not an epic story about a guy saving the world from an evil goldfish, it’s just a guy who has an office job and cold coffee.
You also throw off your flow before you’ve even had a chance to establish it. You leap from back whenever the evil dude was to the modern day. It doesn’t flow smoothly from one scene to the next; it’s jolting. It could work if you moved from the gripping action into more action that then triggered the flashback.
For example, the author opens with a flashback where Davina used to be a superspy, but something went wrong, a bomb went off and killed her wife. It scarred her for life. She then hears ticking, sees the tell-tale wires, and hears the maniacal laughter of the evil guy with the fez; it’s happening again. She snaps from the flashback, into the present, tries to pull herself together to save the day and do it right this time. Then, it’s less jolting because the overall tone and setting, while separate and distinct, are related. It gives some idea of the character, and you don’t lose that tension and action. You’re not cheating the reader.
Moving on from the opening scene, you could find that the character keeps having flashbacks and nightmares relating to a big event. That again throws off the flow, and if the action is that exciting, the emotional torment that great, don’t write that event as a flashback, write that event as the book.
You want your book to move forward, not backward. There will be room for some flashbacks, memories, nightmares, whatever form takes your fancy, but they can’t stop the overall forward progression and flow. They need to add to the plot, and the character needs to progress onwards from that bad, horrible experience. If you keep focusing backward, then the book will jolt at best and drag its heels at worse. You risk losing the reader’s interest, as they’re likely to start shouting, “Argh, just tell me what happens! Why didn’t you write that book instead?”
Ok, maybe that last little bit is just me, but my point stands. Use the flashbacks, etc. carefully, don’t rely on them, and if you have to keep going back to that event, just write that book instead.