If there is a reduction in performance quality on a given show, either the performers are not applying themselves with the same intensity (as I suggested), or there is some other explanation.

Whatever factors, including intensitity of actors applying themselves, affect the quality of performance, themselves will have some distribution that can be mapped to a distribution of performance quality. If a particularly good performance occurs because all the actors had a good previous day, got good sleep the previous night, and applied themselves with full intensity, then the following performance is not likely to repeat that confluence of favorable factors and therefor is not likely to repeat the same quality of performance.