There is a primal relationship between a theatre audience and the actors because of their physical proximity and the power the audience has to affect the actors' performances. This immediate nature of theatre is why it often is called

This immediate nature of theatre is why it often is called "the living stage" or a "living art".

Explanation:

The excerpt in the question was taken from the book "The Art of Theatre: Then and Now"  by William Missouri Downs, Lou Anne Wright and Erik Ramsey. On page 23, they talk of the audience in theaters. That's when the New York Times critic Margo Jefferson is mentioned, and the excerpt is introduced. It is Jefferson who says there is something primal in the audience-actor relationship. By the end of the excerpt, the authors say:

This immediate nature of theatre is why it often is called "the living stage" or a "living art".

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