on Live Write Thrive:

Writers are hounded with the admonition to “show, don’t tell,” which is not easily done well. Showing requires learning masterful technique that can make readers feel as though they are in the story, experiencing it through the characters, moment by moment.However the “don’t tell” rule does not mean that essential elements of a story should be left out. Readers translate the words we write into images, feelings, voices, etc. If we don’t actually put those words on the page, there’s nothing to translate—nothing with which to bring our stories to life.

While showing is the optimal objective in writing fiction, there are times when it’s not enough. Certain things must be explained, and some bits of backstory are needed to give clarity to the action or motivation of a character. Readers can get lost while “watching” a scene if they don’t get why it’s playing out the way it is. Characters might appear and disappear from a scene; motivation may be mysteriously lacking; and actions might erupt out of nowhere. Not good.

What results is sketchy writing.