This thesis, comprising four empirical studies, investigates the process of co-performer empathy in expert ensemble playing. Following an extensive review of the existing literature relating to both optimal experiences of performance and empathy, it begins by probing the relationship between ensemble musicians’ optimal experiences of performance and their experiences of co-performer empathy through a series of focus group interviews. In addition to co-performer empathy, spontaneous interpretative flexibility (SIF) in performance is identified to be a central feature of optimal experiences of expert ensemble performance. Through observational case studies, involving video-recall, acoustic analyses, and heart-rate measures, a model of process of co-performer empathy and the related process of SIF is constructed. The final model shows co-performer empathy to be a cyclical process grounded in a pre-requisite shared approach, both to the music and to working together. It is often characterised by a special connection between players. It involves the identification of a co-performer’s expressive intention, followed by an appropriate expressive response. Co-performer empathy appears to be a context-specific form of musical empathy that emerges as a group process during ensemble playing, and does not seem to be directly related to trait empathy. Finally, from the findings of these empirical studies, potential techniques for strengthening co-performer empathy and the production of SIF in ensemble playing are proposed
