The “show, don’t tell” rule is the very heart of good writing. It’s difficult to avoid, especially when writing a memoir or some other type of nonfiction. But rather than saying a character is unselfish, show them doing something unselfish. If the story is told well, the reader will figure out that the character is unselfish without having to be told he/she is. Readers want to do some of the work. They don’t want to be told what to think, they want to think for themselves and make their own discoveries.
The same is true of settings. Instead of simply writing “the diner was filthy”, a good writer will write something like, “I walked into the diner and was immediately assaulted by the stench of old meat. A hostess approached me who was too old to still be working, forced a smile that unintentionally revealed unfathomable world weariness, and led me sullenly to a formica table with chipped edges. As I sat and slid into my seat, a rat scurried over my foot. I looked under the table just in time to see its tail disappear into a ragged-edged hole in the ancient drywall.”
One might argue that writing “the diner was filthy” is just fine because it says it all, it’s brief, and the reader can just fill in the rest with his/her imagination. The problem is it’s boring. It’s also lazy. It doesn’t engage the readers’ senses or put them in the room. It doesn’t inspire the imagination or pull them into the story.
Besides, it’s a heck of a lot more fun to paint a vivid picture and transport yourself and the reader to another world; a world they can see, hear, smell, touch and taste.
But here’s the catch – always leave more for the reader to discover for themselves. Don’t spell everything out too much. As Stephen King said, “Description begins in the writer’s imagination, but should finish in the reader’s.”