Music therapy provides a complex and dynamic environment and as such offers space for the emergence of nonverbal dialogue. We aimed to capture the relational, musical, and physiological aspects of intersubjectivity emerging during dyadic piano improvisations between music therapists and adult participants in a non-clinical feasibility study. Various data from 17 partner improvisations with adult participants and music therapists were collected, including audio, video, and MIDI recordings, physiological (ECG and EDA) measures, questionnaires, and semi-structured debriefing interviews. Parts of these data sets served for a mixed methods analysis to show (1) how instances of intersubjectivity were experienced by therapists and participants (drawing on interview data), (2) how their shared experiences were supported by coordination and contrast at the level of musical features (drawing on musical data), (3) how physiological arousal was compared between an initial resting baseline and the improvisation, and (4) how patterns in the event related electrodermal activity data related to qualities of intersubjectivity and musical interaction in an example of two case studies. In conclusion, intersubjectivity was characterized by overlapping experiences of relational qualities and complex patterns of musical interaction, including contrasting elements of convergence (in pitch contour) and divergence (key velocity and timing). Further, the case studies showed how differently-interpreted experiences can emerge from a homogenous musical structure, and how stronger intersubjectivity may be associated with more active experiential phenomena such as disruption or reciprocity (versus relaxation or security). Additionally, our findings highlight the complexity of musical interaction as an enactive process. Bringing together multi- and interdisciplinary perspectives enhanced a shared understanding of how subjective experiences that arise during dyadic improvisations relate to measures of musical coordination and physiological arousal. The complex mixed methods design proves to be feasible and, as a consequence, can be transferred to clinical settings in the future.