More film terms, yay!
All matter wants to exist in a state of low entropy. Aspects of fiction (especially long running or adapted fiction) is no different… as long as the writer wants it that way.
“Broad Strokes” is another way of saying “canon is upheld as the plot demands.” Often, deviations from that canon are on purpose (after a lot of time has past) but it doesn’t always feel that way.
Most uses of this in shorter form works are adaptations of books or reboots of older films where the way the new writers want to use it doesn’t call for some aspects of the previous canon, or time constraints don’t allow for certain details. What aspects of canon they do take are the “Broad Strokes” and the rest are washed in the swath. The JJ Abrams helmed Star Trek reboots are pretty recognizable examples of this in action.
Broad Strokes generally happens in longer form works when some aspect of canon is retained, but a more specific aspect of that canon is ignored in favor of some factor.. usually drama and stakes, or time. Stakes are hard to maintain anyway, if you’ve got some glaring aspect of the stakes that rendered them useless in a previous episode because “drama” at the time, but you need this particular stake, then “drama” is also the perfect excuse to just.. sorta… ignore that compromising factor and move on.
When a show lasts a really long time, and pretty much everything had some sort of further definition that seemed great at the time of addition but doesn’t really work so well right now for whatever reason(or never really did but it was used anyway to get the desired result), it’s often lost to the great swath of broad strokes. Pretty much every episode of a tv show is an adaptation of previous versions.
This is where “flanderization” technically comes from over time: A character’s quirk eventually takes over their personality and everything else gets broadstroked out except the lowest form of recognizably.
Serial dramas tend to fair better at retaining their nuance, but writers still use this technique quite a lot. It’s not always easy to write certain necessary scenarios, but sometimes tools like Broad Strokes can be used to retain the needed effect even if some aspect of consistency is lost. Sometimes that loss of consistency is the point. It depends on the scenario and the writer whether they can pull it off.
A variant of Broad Strokes is also used in symbolism since it’s supposed to be simple and traceable. An example of this is “Dean probably made that mixtape for Cas.” Yes it’s possible that he made it a long time ago, yes it’s possible that it was platonic in nature, but those are both unlikely because Broad Strokes. The complicated bits get lost in favor of maximum emotion. Whatever the most dramatic thing is, that’s probably the answer.