American representative democracy

For American representative democracy to function as intended, a citizen (principal) has to select a good representative (agent) to act in their stead and pursue the citizen’s interest (s). Reciprocity is a key element in this relationship. Broadly speaking, what would agents/representatives need to know about the principals/citizens in order to represent their interest well? Are there certain characteristics that would make someone better suited to serving as an agent/representative? What are the responsibilities of the citizens or principals in this relationship?