Automatic accompaniment systems are comprised of realtime score following systems that in reaction to recognition of events in a score from a human performer, launch necessary actions for the accompaniment section. While the realtime detection of score events out of live musicians' performance has been widely studied in the literature, score accompaniment (or the reactive part of the process) has been rarely addressed. In this paper, we expose the respective literature concerning this missing consideration. We show how its explicit design considerations would allow correct accompaniment despite machine listening or human errors introduced during score following, and furthermore how it enables a more elaborate authoring of time and interaction for mixed live electronic pieces.