A crowd of people board a subway car and discover nazi symbolism written everywhere. One person stood up and started cleaning it off. His courage enabled everyone else to stand up and help until all of it was gone.
This would make for a great class discussion on the role of conformity (the Asch study), which showed that conformity drops dramatically when only one person decides not to go along with the group's decision.
Personality: what kind of personality traits would make some most and least likely to stand up and take the first action? The Big Five traits:
Openness to Experience
When all the passengers first sat down and saw the graffitti they all looked at each other and nobody did anything.
Here's a good point made by the video below: while being the first to take action is important, being second is also really important too. Once two people have taken action, most of the rest of the crowd will now feel more emboldened to do the same thing.
NOTE: the topic of conformity is discussed in Nevid (Essentials of Psychology) on pages 475-476.