This paper suggests that interlocutors categorize speech events in terms of context prototypes, providing a device for capturing both regularities and deviations from a norm, within a system where linguistic behavior varies according to context fluctuations. The analyzed dialogue illustrates that shared identification to a social norm has its limits. In particular a successful Instruction-Giving activity type is shown to depend upon the instructee's acceptance of unequal status. It is also demonstrated that differences in language competence can generate non-prototypical speech events.