Source : Webster's Revised Unabridged Dictionary (1913)

Byplay \By"play\, n.
   Action carried on aside, and commonly in dumb show, while the
   main action proceeds.

Source : WordNet®

byplay
     n : incidental activity performed by an actor for dramatic
         effect; "his business with the cane was hilarious" [syn:
         {business}, {stage business}]