Understanding audience responses to art and performance is a challenge. New sensors are promising for measurement of implicit and explicit audience engagement. However, the meaning of biometric data, and its relationship to engagement, is unclear. We conceptually explore the audience engagement domain to uncover opportunities and challenges in the assessment and interpretation of audience engagement data. We developed a display that linked performance videos with audience biometric data and presented it to 7 performing arts experts, to explore the measurement, interpretation and application of biometric data. Experts were intrigued by the response data and reflective in interpreting it. We deepened our inquiry with an empirical study with 49 participants who watched a video of a dance performance. We related temporal galvanic skin response (GSR) data to two self-report scales, which provided insights on interpreting this measure. Our findings, which include strong correlations, support the interpretation of GSR as a valid representation of audience engagement.