A single, carefully chosen detail that reveals more about a character, setting, or situation than a full paragraph of description ever could.
A telling detail is a specific, concrete detail selected not just for what it shows but for what it implies. Instead of describing everything about a room, you choose the one detail that matters: the single unwashed mug in an otherwise spotless kitchen. Instead of cataloguing a character's appearance, you notice the way she keeps touching her ring finger where a band used to be. The power of a telling detail is in what the reader infers from it.
New writers tend to over-describe because they're afraid the reader won't see what they see. Experienced writers trust one perfect detail to do the work of ten adequate ones. A telling detail respects your reader's intelligence - it gives them a puzzle piece and lets them assemble the picture. That's what makes readers feel smart and engaged rather than lectured at.
The detail of Gatsby's shirts - piled high, thrown into the air, making Daisy cry - tells us everything about wealth, performance, loss, and longing in a single scene.
Capote notes that Nancy Clutter taught a neighbor girl to bake a cherry pie on the afternoon before the murders. That ordinary, kind detail makes the violence that follows unbearable.
Carver's minimalist style depends on telling details. The way the narrator watches his wife's face while she listens to the blind man's tape recordings tells us everything about the marriage without a word of exposition.
A telling detail works because it stands alone. If you describe the ring finger, the shoes, the posture, the haircut, and the watch, none of them are telling anymore. Pick the one that matters most.
A telling detail must be specific. 'She wore old clothes' isn't telling. 'She wore a concert t-shirt from a band that broke up in 2003' is. Specificity is what makes the reader's imagination fire.
If you write 'the unwashed mug in the spotless kitchen, which showed she was too distracted to clean it,' you've killed the detail. Present it. Trust the reader.
Describe a character you know well in exactly three sentences. The first sentence must contain one telling detail about their appearance. The second must contain one telling detail about their environment. The third must contain one telling detail about a habitual action. No adjectives about personality or emotion allowed - let the details do all the work. Then ask someone to read it and tell you what kind of person they picture.