This post relates back to my post on plot holes. Not all of them need to be filled, but when they do the how of it is important.
Plot spackle is a failure state.
When trying to fill a plot hole, to explain away some oddity in your world, take the opportunity to add something interesting to your book. We can tell when you’re just filling in something at the last minute. When and if you find a problem with your story in editing, it frequently means that you’re going to need to change something fundamental. Most things big enough to qualify as plot holes involve fundamental flaws in a book that need repairing. This isn’t always true; the Death Star being big enough that crossing it would take over a dozen hours on a commercial jet and yet the cast finding everything they need within walking distance is immediately solvable by simply making it smaller. However, the question of why Hermione could possibly [long dead spoiler alert] develop feelings for the whiny and insufferable cardboard cutout of dear Ronald is something that’s going to take overhauling.
Now, we could just say that she has appalling taste due to some magical mishap in her early childhood which overwrote her ability to distinguish quality men from chip sandwiches, but no one is buying that. No, we need something a bit more substantial. We need to alter her entire romantic arc to include someone remotely in her league, make Ron more sexually alluring than your average golden retriever, or create a wizarding world in which pairing occurs based on something other than looks, money, personality, or social standing; play to Ron’s strengths, whatever unseen virtues those might be. In other words, something big has to change early on in order to make the outcome more plausible.
All that being said, there are some circumstances where it’s just too painful to fix something at a more fundamental level. Whether it’s because you love the early parts too much to tamper with them or it’s just a carefully-constructed house of cards that can’t stand the tremor of even a subtle shift, it does absolutely happen. The failure state, then, needs to be done so well that it becomes a strength rather than a weakness. That’s remarkably difficult, but not impossible. The key to it is to make your last-second repair interesting and thematically consistent. It needs to add something to the world in and of itself, not just a repair but a feature.
The best example of this I ever saw was the way Orson Scott Card, in Ender’s Shadow, filled the problem of why a scientist whose research had been wiped from the world for security reasons was wandering around to be questioned (as was necessary to advance the plot). Card combined three things:
Firstly, he added a clever technological feat by explaining that the scientist’s mind had been conditioned to have a panic attack whenever he spoke on the forbidden subject.
Secondly, he added a fascinating character by making the scientist a man who lived his life in a constant state of amusement to avoid the chance of accidentally triggering the attacks.
Thirdly, he added an insightful parallel between the science for which the man was crippled, itself central to the plot, and Abrahamic religion, an idea which had a profound effect on the nun who was doing the questioning.
In one plot-hole-filling exercise, he gave us an entire chapter that stands as one of the best in his text and which is only apparent as spackle to those of us who make a living looking too closely at things. Therein we find the key to spackling if you need to. Finding a way to fix a major problem with a minor change that still brings something to the book is hard, and it’s harder the more you love the book and the closer you are to the issue. An editor can take you a long way on that front, and if you can find a qualified beta reader, that might help as well. Of course, if you find a qualified beta reader, hold on to them, because the odds are you are the only one in the world to have done so.
So, to conclude: Avoid spackling at nearly any cost. If you absolutely must spackle, do it so well, so artistically, that the beautiful rug in the middle of your living room becomes more important than the gaping, half-repaired maw it conceals.