The intrapersonal functions of private speech have been researched extensively. However, studies in interactive settings that focus on the interpersonal functions of private speech have been rare. Adopting a case-study approach, I investigated the social consequences of private speech and how it contributes to the establishment of intersubjectivity during collaborative tasks. Data were collected through video recordings of participants collaboratively processing feedback on jointly produced texts, retrospective interviews that included stimulated recall, and collaborative writing tasks. Findings show instances of private speech acting as a tool that helps establish intersubjectivity, thus promoting joint problem solving and potentially affording interlocutors more opportunities to co-construct knowledge. These opportunities for learning were made possible when interlocutors used the private speech of a peer as a resource to build upon when deliberating on a response to feedback. This indicates that the ability to understand the intra- and interpersonal functions of an interlocutors' private speech is an important aspect of interactional competence in an additional language.