It may also be the most useless.
Yes, showing has its virtues, the most obvious of which is that dramatization can have a powerful effect on the reader. To give an obvious example, we are much less likely to be impressed by a writer's description of one of their characters as "intelligent" than by their actually demonstrating that character's intelligence within the story.
But in practice, fiction just about never adheres to this "ideal." Rather than "Show don't tell" it is always a matter of "Show and tell," because showing, like telling, has its limitations, requiring one to resort to the other, less celebrated method.
Chuck Wendig's comparison of the two modes nicely "shows" us the difference between the one and the other:
Telling is explanation. It is definition. It is text. It says, This is that.Of course, revelation and illustration and subtext and evocation trades the quick, sure way of telling the story for a lengthier process, less certain to convey the same information to the reader, who has to work harder to understand what they are reading because of the inferences they must draw from fragmentary images. It means writing which, all other things being equal, is slow and ambiguous and difficult--hardly things we associate with reading pleasure. Indeed, far from achieving that heightened dramatic effect, if the showing bores or confuses or overtaxes the reader, don't count on them to get the point, or care even if they do get it.
Showing is revelation and illustration. It is subtext. It asks, Is this that?
Telling walks ahead of you. It pulls you along.
Showing is the shadow behind. It urges you forward.
Telling invokes. Showing evokes.
And of course, a writer's scrupulously abiding by this rule affects their choice of content in ways that are not always for the better. For instance, an adherence to a simple-mindedly literal understanding of "show don't tell" is apt to push us out of the character's heads, leave us looking in at them from the outside--and in all likelihood, not get to know them as well as we might if we had more direct insight into their inner life. In fact, as Joshua Henkin remarks, they might spend their time describing couches instead.
Naturally, good telling can be a lot more effective than bad showing, as well as considerably easier to achieve. Which seems to be exactly the point. The advice about what makes for "good writing," by and large, reflects capital "L" literary standards, much more than it does what people actually look for in the books they really read.
And capital L literature places a very high stress on conspicuous technical accomplishment. Showing is esteemed precisely because it is hard--and because its downsides are looked at differently. If it is slow that does not matter very much to the Guardians of Good Taste. The emphasis on the perceived outsides of things and all the pitfalls the unavoidable ambiguity creates for observers, is a good in itself--the author's minding his lowly place and not presuming to tell us how things really are because what the hell does he know anyway--is respectably "postmodern," while also not offending against the "discomfort with emotion and sincerity" that Henkin correctly identifies as part of this sensibility. And if the reader has to work that much more, and possibly get less for it--well, the prevailing Literary sensibility is defined by exactly those who enjoy textual puzzles for their own sake.
At any rate, more nuanced advice, which would give us some notion as to when to show and when to tell, like just about everything else having to do with writing, cannot be crammed into a concise, pithy-sounding, one size-fits-all formula. (Henkin, indeed, remarks that "show, don’t tell" is a "mantra between a lazy student and a lazy teacher.")
There is simply no substitute for a well-honed sense of judgment, fitting the mode of storytelling to the specific purpose at hand, one more reason why acquiring the craft can be such a lengthy and difficult process. Nonetheless, the non-exhaustive guidelines given in this post at FimFiction seem a good start. At the very least, tell, don't show, if "your narrator has a distinctive voice," if "your audience doesn't care," if "you want a scene to move fast," or if "you are making an important point or giving information that a reader needs to have absolutely clear for their understanding of the story."
To show rather than tell at such times is to show too much.
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