Ernest Hemingway's The Old Man and the Sea is a spare, tightly focused novel with only one central character alone at sea for most of its length. To open out Santiago's world and reveal his inner life, Hemingway relies heavily on two devices: flashback (memory) and monologue (speech and thought). Both are essential to the novel's meaning.
Flashback deepens character and hope. Because Santiago spends most of the story alone, his memories carry much of the narrative weight. He recalls the great arm-wrestling contest in Casablanca, when as a young man he beat the powerful black man in a match that lasted a day and a night. This memory reassures him of his own strength and endurance at the very moment his old body is failing against the marlin. He also remembers the lions he saw on the African beaches in his youth, a recurring image of vigour, innocence and peace that returns in his dreams and sustains his spirit. Through such flashbacks the reader learns of Santiago's past prowess and understands the pride and stamina he still commands.
Monologue reveals thought and endurance. Alone in the skiff, Santiago talks aloud, to himself, to the fish, to the birds and to his absent friend the boy Manolin. This spoken monologue, together with his running interior reflection, lets Hemingway convey the old man's courage, his strategy in the fight, his moments of doubt and his stubborn determination. His repeated wish, "I wish the boy were here", expresses his loneliness and his need for companionship. His reflections on the marlin, whom he addresses as a worthy brother, reveal his respect for his adversary and his complex feelings about killing so noble a creature.
Combined effect. Flashback supplies the depth of a lifetime behind the present ordeal, while monologue makes the solitary struggle dramatic and intimate. Together they turn what could have been a bare fishing anecdote into a profound study of courage, dignity and the human capacity to endure.
Hemingway therefore uses flashback and monologue not as ornament but as the very means by which a solitary man's heroism is made visible to the reader.