This study was carried out in three secondary schools of varying socioeconomic levels within the usual framework of tasks completed by students while studying towards a national assessment. The study examined the change in the nature of the interactions between students in group conversations, in the context of a film unit and a geography unit using Quality Talk. Quality Talk is an interventional approach promoting text comprehension via a critical-reflective thinking approach. The study also sought to determine whether there was a positive intervention effect from the use of Quality Talk, on the students' ability to write from a critical analytical (CA) stance. Differences in the pre- and post-treatment writing achievement of seven intervention and one comparison class were assessed. Analysis revealed a large intervention effect within the intervention classes with marked improvements in students' abilities to talk and write with a CA stance (d = 0.92). The change in behaviour for the students in the intervention classes appeared to be associated with their increased use of authentic questions, uptake questions and high-level questions, which appeared to foster higher levels of dialogic spells, supporting the development of higher levels of CA talk and writing.