Wiktionary
n. (context theatre English) action carried on aside, commonly in dumb show, while the main action proceeds; action not intended to be observed by some of the persons present.
Usage examples of "by-play".
We have omitted--as a matter not congenial to our taste--the small by-play which had been carried on by the other damsels of the village to effect the same object.
But if you turned off the by-play between Mission Control and the Sea of Tranquility, with its deliberately mundane and routine chatter, and stared into that black-and-white television monitor, you could glimpse that we humans had entered the realm of myth and legend.
At least, he thought, he was semi-human enough not to inflict further damage upon her amour propre by telling her that the last little bit of by-play had been purely for the benefit of Branson who had momentarily lost interest in the screen-he was not then on camera-and was casting a speculative look at them.