This paper explores patterns of interpersonal behaviour amongst teachers and pupils during one-to-one instrumental lessons. It was hypothesised that these patterns might differ in systematic ways, according to an existing model of six interaction "types" developed within a systems theory perspective and based on measures of interpersonal control and responsiveness. Eleven violin teachers and their pupils aged 10-16 were observed. Digital recordings of 23 one-to-one lessons were analysed using the overarching categories of teacher talk, teacher questioning, scaffolding, pupil talk and pupil playing. Behaviour style was found to correspond with the model of interpersonal interaction in instrumental teaching. Highly directive teachers were found to engage most often in scaffolding, while the most responsive teachers allowed space for the pupil voice to be heard and provided more feedback that was attributed to specific strategies or effort. These findings suggest that teachers and pupils may become entrenched in fixed patterns of interaction behaviour that potentially place constraints on teaching and learning outcomes. The implications for practice lie in teachers' awareness and reflections on their own interaction behaviours. Being cognisant of fixed patterns of behaviour, teachers may choose to re-frame their style of interaction, thus effecting positive changes in teaching and learning.