Recent years have witnessed the ethnographic turn in academic writing research and the multimodal turn in classroom discourse studies. Most research proposal genre studies have only provided generic guidelines, and most EAP intervention studies have only provided pre- and post-intervention learning outcomes but have not examined actual in-depth classroom teaching and after-class consultative processes of proposal development. This paper fills these gaps by following the view of genre as contextualized social practices and adopting interactional ethnography (IE) logic-of-inquiry to analyze video-recorded linked EAP classroom interactions and WeChat-mediated individual consultations focusing on postgraduate students' topic selection. IE-guided reasoning and analysis may make the invisible research topic developmental processes transparent by utilizing IE analytic tools to visualize these dynamic and complex processes, identify cultural patterns of academic literacy practices, and (re)theorize educational phenomena. The IE approach may complement the well-established three approaches to student writing (i.e., study skills, academic socialization, academic literacies) as the fourth approach for its deep theorizing, micro-ethnographic discourse-based analysis, and video-enabled IE-guided analytic tools. This study may make a methodological contribution to the EAP field through the IE as an epistemology with analytical tools and propose a tri-level four-step IE analytic model for teacher-researchers' self-reflexive professional development.