We are assuming the roles of people who are long dead. It seems ironic that such a point should be made in a play, where characters must remain consistent in themselves and in their relationships for the course of the work. Their relationships and their characters change and evolve, but representationally they remain the same people. A single actor plays a single character for the whole of two hours. A play s survival depends on plot and structure.
Time is very important, and one event triggers another sequentially.
The relationship between two characters in the first act directly shapes their relationship in the second act. The audience does not disregard what has passed between these characters earlier as an interaction between two dead people. And certainly neither does the playwright. But the irony here is only a deception. Eliot deliberately chooses the form of the play because it serves as a model for the way most people proceed through their lives.