Teachers' development can be seen as a dialog between their reflective thinking and their actions, with an added gradient of complexity from their social interactions. All of these elements are conjugated within their classroom practice, with their Pedagogical Content Knowledge (PCK) as theoretical background. This paper analyzes the case of a secondary education science teacher's development over different periods, with a focus on classroom atmosphere, social organization and problem-based interactions, the influence of the topic being taught, and the teacher's participation in an Action-Research group. The sources of information included the teacher's diaries, questionnaires, interviews, ethnographic records, and extracts from videotaped sessions of her lessons. The data analysis for the areas of reflection and of action was approached with methodological plurality. Although these two fields of study (reflection and action) share essential aspects, they presented subtle differences, with reflection being more fully developed than action in the classroom, and the contribution of the Action-Research group to the teacher's development was important, but less so than her professional command of the content. Two underlying obstacles deeply rooted in the teacher's thinking and actions impregnate her classroom interactions: classroom competitiveness, and the use of excessively rigid activities.