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.
Super Trope of:
- Alleged Lookalikes
- Dawson Casting: The audience is told that the 20-something actor is actually a teenager, and goes along with it.
- Designated Hero: The audience is told that he's good guy, but we see them as jerks at best and outright villains at worst.
- Designated Love Interest: Two characters are said to be in love, but their interactions don't support that.
- Designated Villain: The audience is told that he's evil, but we don't see them doing anything wrong.
- Faux Action Girl: A female character is said to be strong and badass, but never shown in any situations that would require those attributes.
- The Friends Who Never Hang: Two people within a large close-knit group who aren't especially close to one another.
- Have I Mentioned I am Gay?: The character is said to be homosexual, yet they never display any sort of romantic feeling or sexual attraction toward characters of the same sex.
- Humor Dissonance
- Informed Ability: A character is stated to be skilled in some area, but either doesn't use the skill at all in the story or does so in an unimpressive way.
- Informed Attractiveness: The characters talk about how attractive someone is.
- Informed Deformity: Characters talk about a certain body part of that particular person that looks a certain form yet he/she displays no visibly weird form.
- Informed Equipment
- Informed Flaw: This character has a flaw, but it's never shown.
- Informed Judaism
- Informed Kindness: The person is really kind? His on-screen attitude says otherwise.
- Informed Location
- Informed Loner
- Informed Obscenity
- Informed Power
- Informed Poverty: This person is poor yet can somehow afford things poor people couldn't.
- Informed Species
- Informed Wrongness: We're supposed to side with one person because they say that the other person is wrong, but the logic of the situation makes it clear that we're supposed to support the other person.
- Offstage Villainy: Informed evil deeds.
- The Pirates Who Don't Do Anything: An Informed Profession.
- Underage Casting: The audience is told that the rather young actor is in a profession that requires a lot of education and experience, and goes along with it.