This paper presents an exploratory study of two consecutive reading sessions conducted in primary school by two different teachers. Our purpose is twofold. From a theoretical viewpoint, we propose a tentative set of conditions of teaching effectiveness by relying on the Joint Action Theory in Didactics. From a methodological viewpoint, drawing on a conjectural paradigm, we attempt to articulate various types of analysis (didactic analyses, speech analyses, statistic analyses), in order to test the putative conditions. We consider two generic factors in teachers' effectiveness: how they manage to expose students to the core meanings of the relevant content; and how they maintain a relevant dialectic between uncertainty and certainty in the argumentation process in order to increase the deliberative quality of the constructed certainty.