This article concentrates on a practitioner-driven approach to teacher education aimed at advancing ESL teachers' instructional knowledge about writing. Based on activity-centred interaction between a group of Malaysian English teachers and their teacher educator, the discussion demonstrates (1) the emergent, unplanned nature of strategic mediation in teacher education, as well as (2) the evolution of teacher thinking in the context of ESL writing instruction. Three forms of strategic mediation are illustrated: drafting teacher participation in challenging instructional circumstances; supporting teachers in crafting working definitions for key concepts; and establishing instructional ground rules for carrying out writing lessons in ESL classrooms. Additionally, the article discusses how a three-part activity structure formed the basis of demonstrating, implementing and reflecting on writing lessons, and how this structure paved the way for teachers to reshape their ways of thinking about ESL writing.