Definition of "byplay"

[...] there had been some amusing byplay with the enginemen as to what might be expected to happen if the engines failed to behave themselves properly.

1959 October, Cecil J. Allen, “Locomotive Running Past and Present”, in Trains Illustrated, page 477

The father-son byplay is occasionally overdone and tedious, but there is a genuine warmth in both the Oland-Luke and the Toler-Sen Yung teamings.

1983, Frank Northen Magill, Magill's American film guide, volume 1, page 647