Understanding audience engagement requires methods that capture both subjective experience and embodied responses during live performance. This chapter explores the potential of heart rate (HR) and heart rate variability (HRV) as physiological indicators of audience engagement and flow in theatre. Drawing on research linking flow to the dynamic interplay of the sympathetic and parasympathetic branches of the autonomic nervous system, we argue that HRV provides a valuable measure of the balance between arousal andrelaxation that characterises optimal aesthetic experience. Rather than reflecting stress alone, physiological responses during performance may reveal states of focused attention, emotional absorption, and adaptive regulation associated with immersive spectatorship. We examine how HRV, alongside qualitative audience data, can contribute to a richer understanding of individual and collective experiences of live performance. Building on previous work in music performance and group flow, we consider how integrating physiological measures with analyses of dramaturgical and performance demands offers new possibilities for investigating audience synchrony, emotional engagement, and shared theatrical experience. The chapter argues that combining biometric and qualitative methods advances theatre audience research by providing a multidimensional framework for studying embodied spectatorship and the dynamics of collective engagement.