A group of mathematicians and mathematics educators are collaborating in the fine-grained examination of selected "slices" of video recordings of lectures, drawing on Schoenfeld's Resources, Orientations and Goals framework of teaching-in-context. In the larger project, we are exploring ways in which this model can be extended to examine university lecturing. During this process, we have identified a number of lecturer behaviours. This article describes one of these behaviours where, in what appears to be an internal dialogue, lecturing decisions are driven either by the mathematician, or the teacher, within the lecturer. We analyse two scenarios. In the first, the teacher prevails and in the second, the mathematician. We analyse these behaviours against the framework of the lecturers' Resources, Orientations and Goals that were either stated by the lecturer or inferred from lecturer practice or discussion and subsequently confirmed. The value of the approach used as a professional development model is also considered.