I wanted to ask this question after seeing this question. The two question are similar but kind of touch on somewhat different topics.
Specifically, I am wondering what one should do about "fake" filler scenes. That is, scenes that on the surface look like filler, but are necessary for the story in order to make the plot make sense. Typically these often add important foreshadowing, reveal plot-critical details about the characters, or reveal key details that make later developments make sense. They will often seem banal at first glance, but if these scenes are removed the story falls apart. These kinds of scenes are done easily and frequently in television or long-running comic format, as there is more for filler, but cannot easily be done in movies or books.
A good example of "fake filler" might be the episode "Riddle of the Sphinx" in the animated series Generator Rex. In this episode the heroes fight what appears to be a one-off villain that seems to be part of a simple monster-of-the-week story unrelated to anything important, only for the very last scene to reveal that the monster was actually created by one of the series' overarching villains and thus important for the main story.
Or, this is another example from my own writing (and shows how it kind of falls apart outside of television). I have two plot elements that involve significant betrayal, one in which a character feels torn between their conflicting loyalties and the chaos that erupts when said loyalties are exposed, and another where a character posing as a parental figure turns out to be the story's primary villain. However, unless those emotional connections are established beforehand, which are the type of character developments that generally take several subtle scenes instead of a few big dramatic flourishes, they have no meaning to the reader and it comes across as telling, not showing.
- In the first case, the plot revolves around a villain character (not the villain, but a high-ranking minion) who infiltrates the heroes but ends up falling in love with one of the protagonists. The climax is basically the truth gets out and everything descends into chaos. However, unless there are sufficient scenes establishing how these characters feel about one another ahead of time, the reader feels like they are being told how the characters feel about one another, rather than shown, and thus the events of the climax have no emotional resonance. The scenes are important, but they're subtle character-building and not action-heavy in an otherwise action-heavy story and put the "action" part of the plot on hold. There are few story beats between "villain infiltrates heroes" -> "villain and protagonist get a crush on one another and villain becomes conflicted" -> "villain's true nature gets revealed". I thought about adding some B plot to keep the tension up while the romance arc and character development happens in the background but the problem is...those scenes would be actual filler. As in they would only exist to waste the reader's time long enough for the characters to start developing a budding romance.
- In the second case, the character is the villain the book revolves around. Their actions effectively drive the conflict, but until they betray the heroes there is no conflict. Yet they don't exist until the book begins, so unless there is some buildup to flesh out their relationships with other characters their betrayal has no emotional weight to the reader. Starting the story with their betrayal results in a "who is this and why should we care" reaction from the reader, so it's not possible to move the beginning there to "start as close to the end as possible", because there's no context. I considered introducing a "B villain" group to act as antagonists until the reveal but that effectively inverts the problem, now you have a secondary villain group whose actions are meaningless because their whole purpose is to distract the reader until the real antagonist is ready to move, and would play no further role in the plot.
Notably, these characters are in different stories, so the two cannot be put together to solve the issue.
Given this, how do you handle more subtle scenes that might halt significant action and come off as filler but overall are necessary foreshadowing or character development for the plot to work?