This article explores the following hypothesis: cinema can make us see and hear the ethical and political dimension of musical improvisation. It concretly describes the elements of this politics of improvisation by analyzing several sequences of the film Step Across the Border (1990) by Nicolas Humbert and Werner Penzel. It studies two relational principles that structure the practice of musical and cinematic improvisation explored in the film. Involvement (the integration of the creator in the sound space) and resonance (the mutual transformation of sound, of the context, of the listener) modify the dynamics of the film’s shoot and underline the musical potential of every day spaces (the street, the café, the sea, the factory). Furthermore, the relational power of editing creates a resonant space that blurs the borders between the musicians (Fred Frith and his friends), the film-makers and the spectators. This “experimentation of new social forms of creation” (Saladin 2014, 205) produces a free and singular cinematic experience: in this film, music appears as a plural phenomenon that can shift our gaze, our listening, and thus interrogate our ways of interacting with others.