It occurs when the hero is in trouble and suddenly gets help from a person whom he has never met before and whom he'll probably never meet again.
It can be anything. The stranger can give the hero money if he really needs to buy a train ticket, or he can give the hero the train ticket and say something like, "Oh, I don't need it anyway. Please take it." The stranger can hide the hero in his basement for a few minutes, or lend him a pair of trousers. It can be anything, any possible example of human kindness from a person the hero meets only once in his life.
The author usually uses this trope to help the hero in a situation where these three conditions occur:
- The hero is in trouble and only another person can save him
- Secondary characters are in the same trouble or another and can't help the hero
- The story takes place in the area where there is high possibility of meeting other people (city, park, road, etc.).
Notably, the Kindness of Strangers is not the trope used to introduce a new character- it's a single appearance only. If one stranger aids the hero multiple times it's Mysterious Protector. The stranger does not give the hero any Unobtainium or mysterious advice or gifts- that's more the territory of the shop assistant in The Little Shop That Wasn't There Yesterday. the Kindness of a Stranger is usually "passive help" rather than aiding the hero in his fight with bad guys or monsters.
- The Lost Element: Fluttershy is enlisted by Twilight Sparkle and Rainbow Dash to comfort James immediately after their first encounter when he runs into Sugarcube Corner to escape the prying eyes of the townsfolk after Rainbow Dash fell out of the sky onto him. While cautious as usual, James' extremely reserved mood at the time allows Fluttershy to gently take the initiative to establish a friendly conversation for the distressed newcomer. Subverted in that while they do part ways at first, Fluttershy is reunited with James at the end of the day as a friend during his welcome party.
- One Year on Probation: Akira starts to have a panic attack when he gets lost in the city during Golden Week, but lucky for him, Haru finds him and finds him a place to relax and cool down.
- Whitehall University: Harry Percy is introduced checking on Anne after a bad encounter with Brandon, even though they don't know each other.
- Jack Reacher, running away from pursuers, hides in the crowd of people observing the scene. One of the strangers lends Jack a baseball cap so he can hide better. Stranger doesn't say anything and we don't see him again;
- In World War Z, while searching for drugs for his sick daughter, the hero is aided by the pharmacist who gives him the drug and a few points of advice on how to use it. It's the only scene in which we see the pharmacist.
- In season nine of Supernatural, Castiel is human and homeless for several episodes, and is helped by various strangers, most notably a random driver who sees him on the side of the road and stops to check if he's okay, gives him a lift to the nearest gas station, and hands him a few bucks to get a sandwich. Homeless people are also friendly and willing to help him, prompting Castiel to note that people who have the least seem the most willing to share. Unfortunately, one person he mistakes for a kind stranger turns out to be a Reaper sent to kill him.