With a view to studying the development of social relationships between humans and robots, it is our contention that music can help provide extended engagement and open ended interaction. In this paper we explore the effectiveness of music as a mode of engagement using Mortimer, a robot able to play a drum kit and employing a composition algorithm to respond to a human pianist. We used this system to conduct a comparative study into the effects of presenting the robot as a social actor or as an instrument. Using automated behavioural metrics, including face tracking, we found that participants in the social actor condition played for longer uninterrupted and stopped the robot mid-performance less. They also spent more time looking at the robot when not playing and less time looking at the piano when playing. We suggest these results indicate greater fluency of playing and engagement and more feelings of social presence towards the robot when presented as a social actor.