byplay

Any action, carried out onstage during a performance, apart from the main action.

[...] 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.

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.

Any action, by supporting actor.