Background: Long-form dramatic improvisation has been investigated as an accomplishment of emergent creativity among an ensemble of "players," focusing on how the group achieves "group flow" in performance. Methods: This article employs ethnographic methods (focus group, interviews, and video-assisted self-interviews) to investigate the case of a musical theater improv group. The analysis focuses on how the group describes its shared modes of knowing, drawing on the group's history and their interpreted enactment of these modes in an improvised scene. Findings: Improvisation in this group requires two inter-related forms of knowing: Shared Social Practice (SSP) and Collaborative Affective Attunement (CAA), where SSP involves definable repertoires, resources, conventions, and techniques, and CAA involves affective sensibility of in-the-moment responding, or affective attunement. These two forms of knowing develop over the course of a group's history and are entangled in complex ways over the course of performance. Contribution: Through a case study of a musical theater improv ensemble, the paper contributes to ongoing efforts to theorize the relationship between embodied experience, social practice, and affect in group knowing with special consideration for the significant role of collaborative affective attunement.