We present an analysis of the "in situ" support provided to preservice physics teachers (PPTs) enrolled in a Methods for Teaching Physics course (N = 28) at a research university, while working on the design of 20 lessons in mechanics and waves they taught later in the course. The PPTs submitted 4 evolving plans for each lesson, and received guidance and support in two consecutive consultation meetings on each lesson plan (30 min each), as well as written feedback via the course website. Using grounded theory methods,the analysis examines the interactions between instructors and preservice teachers over the evolving lesson plans as captured in the videos of the consultation meetings and written correspondence. We analyze the PPT-initiated and instructor-initiated concerns, guidance, and support to infer and unpack tacit professional knowledge, practices, and beliefs. The emergent categorization articulates the knowledge, practices, and beliefs that inform and guide the design of physics lessons that offer in-depth treatment of concepts, procedures, practices, and epistemological aspects of physics in an engaging and meaningful way for students, and foster active learning. The extent to which these categories are grounded in the discipline is discussed.