In 1987, Lee Shulman and Hugh Sockett had an important exchange in the Harvard Educational Review. Shulman argued for the central role of knowledge in good teaching. Sockett responded that good teaching cannot be understood as primarily a matter of knowledge and skills, because it centrally involves moral action in particular contexts. This essay sharpens the question of whether knowledge-based or action-based approaches make better sense of educational practice, by considering the power of classroom speech to communicate knowledge and to perform actions. The paper first describes M.M. Bakhtin's dialogic theory of language use - which argues not only that speech simultaneously carries content and performs actions, but also that the two functions inevitably depend on each other. It then provides an example of the interdependence of knowledge and action in an excerpt of classroom conversation. The paper concludes that knowledge-based approaches underestimate how deeply knowledge and action interpenetrate in the classroom.