Visit a place where people wait—a bus or train station, an airport, etc.—and watch people for a while. Choose a person who seems unique. Or go to a museum and choose an intriguing painting of a person. Alternatively, you may listen to a radio call-in show and choose one caller whose voice suggests an unusual character. Once you've chosen a character based on appearance or voice, it's time to write.
In 250-500 words, write about this character who, acting out of desire or fear, makes a mistake. The mistake could be large (e.g., a car crash) or small (e.g., saying the wrong thing). How does the character react to their mistake? How do the people around them react? Remember to use the characterization tools we've studied to show the scene and the characters. The trick is to "dance" between action and defining character.