I linked to and quoted Janice Hardy (Fiction University) in my thread on using sounds in fiction. I was impressed with the way she described why/when to use sounds, much better than I was able to. So I read some of her articles.
One was on when to tell. She succinctly says: "When it is better to tell than to show? If showing is going to bog the story down or bore the reader. "
Kind of obvious, but there's so much more. I love this part:
You tell (in an infodump), but it doesn't stop the story and it allows you to relay information the reader already knows. And that's the most important thing to keep in mind when deciding if you should tell or show. Does it help make the story better and keep the reader reading?
You already do a lot of telling in a novel, even if we don't officially call it that. You describe the setting, what someone does, what they say. It is called storytelling for a reason. The trick is to weave your tells in with your shows so the reader never gets the sense that the author is butting in to explain something to them. Tone matters a great deal, as does voice. Stay in a character's voice, and you can almost anything and make it work.
A flat descriptive paragraph with lots of details and no real context for it is usually boring. Even more so if it falls in the middle of something interesting going on. It stops the story and makes the reader do the literary equivalent of watching vacation slides. But take that same set of details and add judgment and attitude from the POV's eyes. Now it's interesting. It's not just stuff.
The room was small and overstuffed with items from all over the world. Vases from France, statuettes from Italy, wooden shoes from Holland. A bold, green and red flowered wallpaper covered three of the walls, with the fourth wall a solid matching shade of green. On that wall sat a carved trellis with silk flowers entwined through it. Toy birds perched on the top like bright lights.
Bored yet? This is just bad telling when you really think about it. The author is conveying details as if they were sitting outside the room with a pair of binoculars. But if we told it in a character's voice…
Chuck gaped. Holy crap, a garage sale threw up in here. Old vases, cheesy statuettes, those dumb clog shoes with the flowers on them. Bad enough every surface had something awful on it, but did she really need that wallpaper? Maybe she didn't know red and green flowers just screamed "I'm way too into Christmas and need professional help." And what was with that trellis with the fake vines and cheap plastic birds?
Same details, same "telling" what the room looks like, but now there's a person there so the details don't feel so "just telling you what it looks like."
You certainly want to show as much as you can, but sometimes a little telling is needed. Don't be afraid to tell when you have to, just make sure that when you do, you're telling it in a way that serves the story and keeps the reader interested.