Final answer:
Aeschylus's most important innovation in the development of drama was the introduction of a second actor, which significantly altered Greek drama by emphasizing character interaction and complex storytelling, as exemplified in his trilogy, the Oresteia.
Step-by-step explanation:
The most important innovation that Aeschylus made in the development of drama was the introduction of a second actor (deuteragonist). This addition transformed the dynamic of Greek drama, shifting the emphasis from the choral narration to engaging dialogues and character interactions, augmenting the dramatic potential and complexity of the plays.
Before Aeschylus's innovation, Greek drama was largely driven by a single actor and the chorus. By introducing a second actor, Aeschylus opened up new possibilities for storytelling, allowing characters to interact directly amidst their personal and thematic conflicts. This fundamental change is a significant reason why the Oresteia, his best-known work and the only complete trilogy of Greek drama to survive, holds such historical and literary importance.
While other elements such as masks, costumes, chorus, and trilogies were integral to Greek drama, it was the introduction of the second actor that crucially enabled drama to evolve into a more intricate and dialog-driven form of art. Other tragedians, like Sophocles, would later build upon Aeschylus's foundation and expand the number of actors further.