Discussion is a practice used in mathematics methods courses to support prospective teachers' pedagogical ideas. Underexamined but central to the development of instructional activities, including discussions of teaching, are mathematics teacher educators' (MTEs') tacit and explicit theories of learning and teaching. We report findings from a self-study of three MTEs' discussion practice in methods courses. Data sources include transcripts of MTEs' dialogic analysis of their discussion practice and evidentiary maps based on instructional artifacts. We argue that whole-class scaffolding serves as a tacit theory informing MTE discussion practice. We support this argument using evidence that our discussion practice was driven by prospective teachers' move toward independence and by layering instructional activities. [For the complete proceedings, see ED630210.]