The Explanation
Strong descriptions of characters are critical to the success of a story. Descriptions are strong when they allow the reader to visualize the character in his head and to have a strong sense of who this character is. An author creates a sharp image of a character not through generalization, but through specific, concrete details. And the details chosen must be the most powerful, the most significant ones.

In order to create realistic characters, you need to observe real people. Reality is a great source of strength for your fiction.

The Exercise
Go to a public place and pick a person you don't know. What do you notice first about this person? What qualities seem to define the person? Stalk the person for a few minutes. What do you notice next? How does the person move? What gestures, habits, expressions does the person use? What might you deduce about the person from these observations? Are there any contradictions inherent in the person and how she presents herself? What are the most significant details about this person?

Write a description of the person. Then leave the immediate vicinity before you're arrested.