Improvisation represents a unique process of social creativity in real time, practiced in widely varying musical contexts with different levels of experience. Yet psychologist have mostly studied the practices of individual jazz soloists with an expectation that shared understanding, knowledge and technical abilities are a prerequisite for group improvising. A qualitative study interviewed six trios of free improvisers (n=18) to illuminate the processes of shared musical improvisation across a range of contemporary artistic practice. Comparison of different members' accounts of events during recorded free improvisations indicated that their understandings of who did what and why converged at some points, notably during relative stasis, and diverged at others. Improvisers anticipated and interpreted musical behaviors of their collaborators with reference to previous shared social or musical experience, but considered that such expectations could, and should, be confounded. Familiarity between improvisers could be seen as helpful in building trust within dynamic and highly uncertain musical contexts, and in allowing a less conscious approach to interaction. Improvisers individually assumed that their group shared certain tastes and asserted that others recognized certain musical material as connected to previous practice together. These ideas of shared tastes and practices could best be understood as constructions within this particular social context, since they were not necessarily consistent across the ensemble. The findings emphasize the fundamentally social nature of improvising: shared understanding is not a prerequisite for participation, but shared experience over time enriches the resources of meaning an individual can bring to their interaction in an improvising group.