Final answer:
The inner dialogue between Santiago and his heart mainly adds to the development of the plot by revealing his internal conflicts, providing readers deeper insight into his character and personal challenges.
Step-by-step explanation:
The inner dialogue between Santiago and his heart mainly contributes to the development of the plot by revealing Santiago's internal conflicts. This internal dialogue is critical as it allows readers to understand the character's motivations, fears, and desires more deeply. Just as Twain uses dialogue to shift from confidence to anxiety within a character, inner dialogue provides insight into Santiago's personal journey and struggles, adding layers to the story's central narrative.
Moreover, similar to how dialogue reveals backstory and character development in a play, Santiago's conversations with his heart serve to communicate his history and personal growth over the course of the narrative. It also establishes his character by showcasing his unique thoughts and feelings, which are distinct from other characters due to his experiences and reflections. The inner dialogue enriches the storytelling by contributing to a robust understanding of character, setting, and plot.