For professionals such as doctors, teachers, or different kinds of counsellors, talking with their clients is a major part of their profession. Professionals and clients give and ask information until they reach a state of mutual knowledge or understanding. This paper argues that for this talk they use a tool, the machinery of social interaction, with characteristics that influence the outcomes of their talk. One characteristic is the normative organization of interaction through which the contribution of one participant puts restrictions on the range of possible follow-up contributions of another participant. This may cause client behavior that does not align with the institutional aims of the professional. A second trait is that interactants have no access to each other’s cognitions such as intentions, interpretations, knowing, and understanding. Professionals should thus be aware that what clients say has a relatively loose relation with what they know and understand.