A successful design of an interactive system requires a clear understanding of human-machine interaction. For the specification of such a system a precise consideration of the users context during each step of the development process is therefore necessary. Moreover, a formal specification method for expressing interaction is highly desirable in order to achieve a precise and continous specification process between the requirements and design stages. In this paper several of the environmental cues influencing the user during the interaction with a system are considered. These environmental cues are modelled using the concept of information resources. Interaction is described using these resources and formally specified by the notation of graph grammars in order to be able to reason about a system, to assess a system wrt important properties and, finally, to achieve a continuous specification process between the requirements and desing stages. This approach will be demonstrated by specifying a safety-critical system concerning the interaction between the pilot and the flight management system on the flight deck of an aircraft.