You've all heard it, usually at a volume ratcheted up enough to shred your eardrums to smithereens . . .
Yeah, yeah. Whatever. For the most part, I heartily agree with this rule. Showing is hands-down better than telling because, hey, who likes to be told anything? That's about as comfortable as having your mom wag her finger in your face.

But (and I've always got a big but) I've discovered that there are some instances in which telling is a must.

5 Reasons to Tell Instead of Show

1. When you're covering a vast amount of time.
Mundane details of everyday life are boring. Move your story forward by skipping them.

2. Inserting a quick summary.
Sometimes you need to report an event because it's important to the story, yet you don't want the story to get bogged down. A small summary is a useful tool to accomplish this.

3. When backstory is crucial to a current event.
Be careful with this one. I'm talking just a few words here, not entire paragraphs.

4. As a transition.
Scenes must be connected somehow or they'll become disjointed in the reader's mind. A sentence or two of telling can accomplish this faster than a few pages of showing.

5. For a rebound.
Secondary characters are necessary because they bring a well-roundedness to the story. That being said, they should never upstage the main personas. A telling line can kick-off a real-time response from a hero or heroine.

I should also mention that telling is employed more often in novellas than in novels because of the constraint of the length. Showing is still high on the priority list, but as Stephen King says, story is king, and if a little telling accomplishes that, then go for it.