This study examines in-service teacher development around a pedagogical practice central to teaching and learning in mathematics education, "leading a classroom discussion." This practice is used as an artifact to support the development of a teacher's professional community. This work views making teaching practices public as central to teacher's professional community in that it can develop collegial trust. Accordingly, this study supports a teacher's professional community through peer observation. The model of peer observation put forth in this study positions a middle school mathematics teacher as a participant-observer taking on the role of a student at her school for three days. This case study describes this participant observation in which the participant was relieved of teaching duties to allow her to attend -- and be a participant in a full schedule of middle school courses. In addition to the teacher participant-observer, the study participants also include five middle school teachers in whose classes the participant-observer teacher took on the role of a student. Through the analysis of the teacher participant-observer's journal and clinical interviews, this study characterizes the presence and development of collegial trust, conceptual change, critique of collegial practice, development of disciplinary understandings, conceptions of the impact of the institutional setting on teaching and learning, and the capacity to understand the students' perspective. Finally, this study presents evidence from a teacher feedback meeting that indicates the development of the professional community.