The primary source of characterization used by Updike is indirect. Specific techniques used in "A&P" to provide the reader with a characterization of Sammy, the protagonist, are listed below.
Thoughts are enormously important in this story. Told in third person, limited, we are able to follow all of Sammy's observations and reasons for his behavior. The story is told from an objective point of view, so we do NOT receive biased judgments from the narrator, nor is a physical description of Sammy important to our overall view of his character.
Physical actions provide insight into Sammy's character, especially when he stands up for the girls and then quits his job.
Dialogue and the judgment of others (how others react to the character) provides us with the final pieces; most of what we learn of Sammy comes from his thoughts, but some information is based upon what the manager says to him, and it is the manager's judgment that foreshadows the change Sammy will go through now that he has taken his first step into adulthood.