We report on the design and deployment of systems for the performance of live score accompaniment to an interactive movie by a Networked Musical Ensemble. In this case, the audio-visual content of the movie is selected in real time based on user input to a Brain-Computer Interface (BCI). Our system supports musical improvisation between human performers and automated systems responding to the BCI. We explore the performers' roles during two performances when these socio-technical systems were implemented, in terms of coordination, problem-solving, managing uncertainty and musical responses to system constraints. This allows us to consider how features of these systems and practices might be incorporated into a general tool, aimed at any musician, which could scale for use in different performance settings involving interactive media.