Although the use of cases for instruction is increasingly popular in educational psychology courses for prospective teachers, systematic research on how to best use them is just beginning. This paper contributes to this growing area of research with a design-based research project that used a comparative case study design. Specifically, we compared case discussions from two different sections of the same educational psychology course for student teachers, each taught by one of the authors. Case discussions in one section were characterized by broad participation but little grounding in psychological theory while those in the other section were characterized by strong use of psychological theory, but much narrower participation. We then used a set of principles for facilitating productive disciplinary engagement to help explain these differences, further testing and developing them. Each instructor's way of using cases had both positive and negative effects on the embodiment of the principles and thus on the nature of the student teachers' resulting engagement in discussions about them. We close by discussing the implications of the study for future research, theory development and the use of case discussions, especially in relation to how they can be reconfigured to better balance student authority with disciplinary accountability. (Contains 2 tables.)