Acting means performance as an actor or actress. The term acting is closely related to the verb "play." Acting may be false behavior on a live stage or film set where an actor is assuming a role written by others, i.e., screenwriters. Acting is also the professional occupation of an actor or actress. In British stage, actors are often called The Players.
Another form is found in impromptu acting where the actors invent their spoken lines. This can be found in small stage venues, street performances and domestic bedroom scenes. Props, lighting and music often help the players develop and assume their roles. Non-professional or amateur acting is found in many venues and contexts, including BDSM roleplaying.
A spanking scene is impromptu, which means it develops from the players' imaginations as the scene unfolds. At a minimum, spankee and spanker make a consious decision on who will be spanked and by whom. The spanker typically provides acting direction by telling the spankee to assume a position in a place, e.g., "Now, go to the bedroom, turn down the lights, remove all your clothes, and bend over the chair." The spanker may appear wearing clothes and holding a paddle. Such behavior meets the textbook definition of acting, albeit amateur acting.
In domestic discipline, the players may negotiate a spoken or written "contract" that spells out the terms of their relationship including a spanking for breaking agreed upon "rules." The terms of such a relationship set up the acting.