This paper integrates theory, ethnography, and collaborative artwork to explore improvisational activity as both topic and tool of multidisciplinary HCI inquiry. Building on theories of improvisation drawn from art, music, HCI and social science, and two ethnographic studies based on interviews, participant observation and collaborative art practice, we seek to elucidate the improvisational nature of practice in both art and ordinary action, including human-computer interaction. We identify five key features of improvisational action -- reflexivity, transgression, tension, listening, and interdependence -- and show how these can deepen and extend both linear and open-ended methodologies in HCI and design. We conclude by highlighting collaborative engagement based on 'intermodulation' as a tool of multidisciplinary inquiry for HCI research and design.