The characters of Charley and Willy are skilfully developed so that they complement each other in several ways.
Both men are members of the same generation, and in similar stages of their adult life. They are both middle class heads of households, and fathers of boys. As fathers they are doting and loving. They also seem to fit the prototype of the "man of the house", as neither Willy nor Charley seem to mingle with their wives as much as they mingle with each other. In all, they share a good relationship as neighbors, and seem to get along despite of their obviously different personalities.
Charley and Willy's differences are mainly based on the way that they develop as parents, and in the way that they choose to help their children develop towards adulthood. For example, Willy instills in his children a credo which suggests that success can only be accomplished by being well-liked and good looking. This superfluous advice results in Biff and Happy's social ineptness and lack of self-worth as adults.
Contrastingly, Charley consistently demonstrates that his best interests are universally bestowed upon everybody. Charley's son, Bernard, emulates his father's discipline and follows his steady ways. As a result, Bernard becomes a successful adult whose basic philosophy of life has been the same since childhood: abiding by the rules produces the best results.
In the end, Willy sees the big difference between his and Charley's life. Charley is a successful man who even supports Willy economically out of kindness for his friend. Willy is, literally, a "has-been nobody", who still lives his life based in mere fantasy; this has led him nowhere. Willy is aware of this, despite of his denial. He is also aware about the difference between his son's life and Bernard's. When Willy asks Bernard "what is the formula" for success, even Bernard hints at Willy's lack of consistency with his own children.
Whatever their differences, the fact remains that Charley and Willy are important to each other. Charley does not want Willy to sink lower than he already has. Willy needs Charley because only Charley understands his true condition. In the end, their differences are obviously superficial: Charley is the only friend present at Willy's funeral, and the person who tells Biff that Willy's dreams can never be underestimated as foolish. Charley respects Willy despite of it all.