Kindness of Strangers is one of those weird events that seldom happen in Real Life. 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. 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." Stranger can hide the hero in his basement for a few minutes. Or lend the hero a pair of trousers. It can be anything, any possible example of human kindness to the person the hero meets only once in his life.
Author usually uses this trope to help hero in situation where these three conditions occur:
- 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;
- Story takes place in the area where there is high possibility of meeting other people (city, park, road, etc.).
Notably, Kindness of Strangers is not the trope used to introduce a new character. Also, one stranger does not aid the hero many times; that is the Mysterious Protector. Stranger does not give the hero any Unobtainium or mysterious advice or gifts; this would be the shop assistant in The Little Shop That Wasn't There Yesterday. Kindness of Stranger is usually "passive help"; it does not involve aiding hero in his fight with bad guys/monsters. It is more similar to asking your neighbor to lend you a screwdriver.
- 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 of 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.