Competition [in a scene] is healthy. Competition is life. Yet most actors refuse to acknowledge this. They don’t want to compete. They want to get along. And they are therefore not first-rate actors.
~ Michael Shurtleff

In most cases, an actors job is to make unbelievable believable and although this may be for a scene it is also in the interaction between another actor this becomes important as well. Most of this is in understanding what he or she is bringing to the table within their performance that helps the next to step up when it’s his or her turn on the stage too. With daily rehearsal and communication it becomes helpful to an actor in building chemistry within each pass as well as in the performance its self.