This case study is part of a larger project which aims to determine the usefulness and validity of a model of a pre-service content and language integrated learning (CLIL) teacher education programme inserted in a Master's degree, whose main pedagogical option is to achieve teacher empowerment through cycles of collaborative teaching and shared reflection. More specifically, the two-fold goal of the study is to describe the nature of the student-teacher's main accomplishments on her teaching practice, if any, as well as on the quality of her reflection on that teaching practice; and to identify and characterise key stages in her developmental process throughout. The analysis adopts an ethnographic perspective and explores fragments of videotaped CLIL science lessons in English/L3 and other multimodal data (student-teacher's journal, academic reports and instructor's field notes) collected in a master's degree for secondary teachers in Barcelona, where Catalan and Spanish are co-official. Through Multimodal Conversation Analysis and Ethnographic Content Analysis, the study reconstructs the developmental process undertaken by the informant throughout one academic year. The analysis traces the student-teacher's progress both in the practical handling of the specific challenges of the CLIL lessons and in her progressive understanding of key issues in the domain of Second Language Acquisition (SLA); it also shows how teaching practice and reflection shape and fuel each other. In addition, it illustrates how CLIL teachers may benefit from tools developed in the field of Applied Linguistics in order to improve their professional skills. (Contains 1 table, 6 figures, 2 excerpts, and 9 notes.)