The processes of eliciting user requirements and formalising these into specifications are critical for the success of highly interactive systems. These processes are still poorly understood, partly because current methods are usually ad hoc and lack any theoretical basis. A number of researchers have used Activity Theory (AT) to refine these processes and have met with some success. To date, this approach has been more useful explaining the processes post hoc. This positional paper proposes an AT method for requirement elicitation and specification definition. The method is sufficiently prescriptive and well formed that it does not require any detailed understanding of AT.