In this article I draw a parallel between the responsive nature of urban environments in today's hybrid cities and improvisation in the performing arts. By drawing on material from the practice and study of improvisation, as well as the humanities and social sciences, I examine the nature of improvisation and its relationship with urban life. Based on this approach, four key positions are proposed as foundational elements for an improvisation-based model of urban interaction design. A brief analysis examines this model in relation to three existing projects, and by using responsive urban lighting as an example, I illustrate how improvisation techniques can be employed to put into action the proposed key positions to guide the design of responsive urban environments.