While substantial resources support EMI faculty development globally, the professional development of EMI faculty developers themselves remains largely overlooked. EMI faculty developers often work in isolation, lacking systematic preparation and shared frameworks for effective practice. Addressing this critical issue, this collaborative case study examines the design, implementation, and impact of a professional development intervention (PDI) offered by the co-first authors (Rieker & Doran) for Fulbright Taiwan’s EMI Advisors working across Taiwan’s higher education system. The study explores how PDI support enabled two focal Advisors (Ah Puck & Wang, co-second authors) to develop intellectual grounding, expertise, and confidence as EMI faculty developers. Drawing on rich data sources including PDI workshop and pedagogical materials, Advisor-generated artifacts, interviews, focus groups, and autoethnographic narrative inquiries, we employed Vygotskian genetic-analytical analysis to demonstrate how foundational concepts (Doran & Rieker, 2025) offered in the PDI catalyzed demonstrable shifts in the Advisors’ faculty developer practices, including lesson observations, individual consultations, and faculty workshops, as well as the reasoning undergirding their orientations to these practices. Through these accounts of conceptual development, we synthesize principles and actionable recommendations for developing faculty developer expertise across EMI contexts, addressing critical gaps in systematic, scalable EMI faculty development.