I often find, in student manuscripts, instances where the writer tells me something, only to go on to show it. And then tells me again what she’s already shown me. (Mind you, I also catch myself doing this.)
THE PARAGRAPH BEGINS: I could smell the man before I saw him.
It was a rough and rugged street smell. Major body odor, not the kind you get from skipping a shower, but the kind you get from skipping many, many showers.
NEXT, TELLING: Rank and rancid.
There’s no need to do this. In fact, it weighs down the prose. The writer here "shows," in fact let's us smell, what this person smells like. We get “rank and rancid,” before the writer tells us it’s rank and rancid because we’ve all smelled such a person before. The description, the showing, conjures this odor. That is good writing. It creates a three-dimensional world.
If you’re skilled enough as a writer to show like this, then you don’t need to go on to tell. You’ve accomplished what we all strive for in showing: to bring the reader into the story, to have the reader smell what the narrator smells.