Flashback and hallucination are both used to provide character exposition in the play, offering insights into the lives and characters of Willy and Biff in particular.
The flashback scenes help to explore Willy's effect on Biff (for good and bad - mostly bad). They depict the ways that Willy's arrogance passes on to Biff, almost as a form of moral instruction. They also depict Willy's pride in Biff and his hopes for Biff's life. Finally, they explain Biff's disillusionment and his break with Willy as a result of the discovery of Willy's infidelity.
Willy's hallucinations help to articulate his ambitions and his vision of success. He imagines conversations with his brother Ben, both reliving actual conversations and conducting new ones exploring ideas of failure, suicide, pride and despair.
In the absence of a narrator in Death of a Salesman, Arthur Miller uses several techniques to provide exposition. Flashbacks to past events are widely used to illustrate the hopes, dreams and aspirations Willy not only had for himself, but for his family, especially his son Biff. When contrasted with the reality of how Willy’s life has actually transpired and that his high expectations for Biff were never realized, the reader gets a strong sense of why and how Willy’s life has begun to unravel. Miller shows a dramatic shift in the dynamics of the relationship between Willy and his sons. Where Biff and Hap once held their father in high esteem, they are now embarrassed by him. This is also illustrated by Willy’s perceived relationship with Howard’s father as contrasted to his relationship with Howard. Willy is in denial that his level of competence has decreased dramatically and Howard relates to him based on his current inability to function effectively in his job. Willy invents an inaccurate self persona with relation to his actual level of success. He maintains a skewed and inflated view of himself in an effort to hide from the truth. However, this façade begins to crumble as Willy must confront the fact that he is losing his grip with reality. Willy also strongly regrets that he did not follow in the path of his brother Ben. Willy’s acknowledgment of his failures and missteps are pointedly represented by his apparent attempts to commit suicide by smashing his car or using the rubber tubing to asphyxiate himself with the gas from the water heater.