Abstract: A computer program specification system is disclosed for interactively creating program specifications responsive to user input. The system includes facilities to respond to changes, supplied interactively, to a program specification without recompiling the program specification, and immediately to display a simulation of those changes. The system also supports the merging of actors, which are software objects comprising data structures and program procedures, and the merging of actor behavior. A user of the system creates a program specification by specifying actors for use in a plurality of scenarios, and operational steps of the plurality of scenarios. The system executes operational steps of a scenario to evaluate the consistency of the specification and actor data. Execution of an operational step comprises simulating the operational step upon the occurrence of predetermined conditions and storing the results of the simulation.