“Every character should want something, even if it is only a glass of water.” KURT VONNEGUT
Think about each character, no matter how minor, as if the scene were viewed from his or her perspective. What does the character want? What’s at stake for the character? What is the character’s history? Age? Temperament? How does the character want to be seen by others? Knowing each character intimately will help you write a character who is real to the reader.
In a sense, writing each character is akin to method acting. Crawl inside your character’s skin and, when you ask the character to say or do something to advance the plot, ask yourself: “What’s my motivation?” You must always be able to answer why this character, in this situation, would behave in the way you have planned for him or her.