While Andy is the main character and the focus of the novel, it seems that readers too glean some sense of identity from Andy's friends. For me, when creating them, I loved thinking about them in real time: What do they eat? What do they wear? How do they walk? What makes them angry? What makes them sad? The list goes on and on.
Whenever I get a role in a play, the same process occurs. Who is this new character? How does he walk? How does he hide his embarrassment? How does he show his happiness? What is his favorite food, song, movie, book? It is a blast using the clues from the script to decipher the character.
Much of that theatrical 'training' becomes the same foundation for creating characters. A great shout out to Ken Hornbeck and Peter A. Carey for some of those theatrical juicy tidbits!