Informed Attribute

"I have yet to have an author inform me that a character is charming, and then, by that character’s deeds and conversation, convince me of that fact."
Dorothy Parker, "These Much Too Charming People"

A violation of Show, Don't Tell when fleshing out a fictional tangible. Something is said to be X, but not shown to be that way.

Making a fictional something come to life is difficult. You only have so much Willing Suspension of Disbelief to work with, and you don't just want this thing to be believable—you want it to have the characteristics it needs to have for the story to work. You want this character to be brave, that knife to be sharp, this table over there to be sturdy, and this means going through a lot of trouble. Think of a situation where the thing can show its true colors, think of a way that this situation can naturally arise out of the plot, think of a believable way for the thing to act and finally show the way it is…

Which is why authors sometimes, consciously or not, go "screw it. Why go through all the trouble?" and just have the narration or another character say, "she is brave", "this is sharp", "this is sturdy". There. Problem solved!

Except, no. Not by a long shot.

Readers do not come to a story expecting to be spoon-fed what they should be feeling or thinking about things. They want to experience it for themselves and decide for themselves. They want the freedom to conclude that the Strawman Has a Point, that The Hero is really more of a villain, that the aesop preached by the mentor is hopelessly misguided based on what happens five seconds later, that the "ultimate" Dangerous Forbidden Technique is worthless.

Deny them this—either by sketchy writing or not doing the research—and there will be backlash. Readers will stop taking your word for it, whatever "it" is. They will actively seek reasons to hate the people you tell them they should love, to downplay the forces you tell them are powerful, to ridicule the concepts you tell them are profound. It's difficult to draw the line regarding where the ratio of informed feats to actual feats becomes unfeasible, but you can trust readers to recognize it when you cross it.

And that is a very good reason—no, scratch that, that is the reason—that "Show, Don't Tell" is a piece of advice drilled into the heads of amateur writers early and often. Master it, and you can start using tools like the Internal Reveal, when a character is shown to be X but doesn't realize it about themselves.

Supertrope of:

A Sub-Trope of Show, Don't Tell. See also Hollywood Homely, Urban Legend Love Life, Have I Mentioned I am Gay?, Overrated and Underleveled. If everyone keeps going on about how great you are, it's Character Shilling.