Show, Don't Tell is an overlooked, yet important fiction writing tip. The idea is to "show" the readers that a character has trait [X] instead of simply telling them. It's a bad idea just to say that "Gary is good at fencing.". Back it up. Have him demonstrate that he is good at fencing by having him win. If the book isn't about fencing, have some medals or trophies in his room.
There are two ways to violate this rule.
- Tell a trait [X], but not show trait [X].
- Tell a trait [X], but show trait [Y].
Saying that a character is sensible but not backing it up is one thing. Saying that a character is sensible and then have them fall prey to incredibly stupid traps is another thing, and should be avoided at all costs.
If an author has trouble with show, don't tell, he or she should consider this. In real life, you learn many things about people from their actions. It isn't likely that they'll be a told that a person likes to read, either by that person or by another. They'll pick it up when they always see them with their nose in a book. Think of how people in real life demonstrate traits and characteristics.