In this paper, we discuss the design and testing of a reactive envi-ronment for musical performance. Driven by the interpersonal in-teractions amongst musicians, our system gives users, i.e., several musicians playing together in a band, real-time control over certain aspects of their performance, enabling them to change volume lev-els dynamically simply by moving around. It differs most notably from the majority of ventures into the design of novel musical in-terfaces and installations in its multidisciplinary approach, draw-ing on techniques from Human-Computer Interaction, social sci-ences and ludology. Our User-Centered Design methodology was central to producing an interactive environment that enhances tra-ditional performance with novel functionalities. During a formal experiment, musicians reported finding our system exciting and enjoyable. We also introduce some additional interactions that can further enhance the interactivity of our reactive environment. In describing the particular challenges of working with such a unique and creative user as the musician, we hope that our approach can be of guidance to interface developers working on applications of a creative nature.