The primary purpose of this study was to design a reliable and valid continuous-time coding tool for measuring teacher use of space and teacher interactions based on prior research (Hesler, 1972; Martin, 2002). The tool captured teachers' use of space as they moved through 14 identified areas of the large instrumental ensemble classroom and engaged in 10 types of verbal and musical interactions with students. Evidence for content validity is presented, and the tool was found to have high inter-rater reliability. The secondary purpose of the study was to explore the effect of a brief expository lesson on preservice instrumental music teachers' use of classroom space and proximity while teaching, with specific emphasis on teachers moving away from the podium and toward and among students. The expository intervention changed teachers' use of space during the lesson immediately following the intervention (i.e., teachers spent less time on the podium and more time moving toward and among students), but the behaviors did not persist over time.