In this paper, co-musicking in teaching is discussed on the basis of findings from a study on pop band and piano teaching. We understand co-musicking as collaboration between music-makers, including pupils. For this study, we chose to focus on the actions that teachers and student teachers take during pupils' performance of a piece of music in a teaching session. The data was gathered using stimulated recall interviews. The findings include the use of different modes: teachers playing along with students, using their voice, or using gestures and body language. The purposes of these actions included scaffolding performances and experiences, recognizing pupils' effort, and opening up a space for reflection. We suggest that the teaching practices studied can be interpreted as improvisational because the teachers and student teachers respond instantly, flexibly, and creatively to the pupils' performances and expressions.