why does hemingway choose to rely so much on the old man's introspective internal dialogue? what does this do for the story?

The introspective dialogue allows the reader to understand and even relate to Santiago. Being heroic and manly are not merely qualities of character which one possesses or does not. One must constantly demonstrate one's heroism and manliness through actions conducted with dignity. Interestingly, worthiness cannot be conferred upon oneself. Santiago is obsessed with proving his worthiness to those around him. He had to prove himself to the boy: "the thousand times he had proved it mean nothing. Now he was proving it again. Each time was a new time and he never thought about the past when he was doing it" (66). And he had to prove himself to the marlin: "I'll kill him....in all his greatness and glory. Although it is unjust. But I will show him what a man can do and what a man endures" (66). A heroic and manly life is not, then, one of inner peace and self-sufficiency; it requires constant demonstration of one's worthiness through noble action.