Crowd simulations are widely used to study and predict the human behavior in disaster scenarios. In this paper, we introduce real-time user interactivity into the simulation process of virtual environments (e.g., buildings with rooms and doors between them). We develop a new tactical path-planning model that translates the interactive virtual environment into an abstract graph in order to calculate the shortest paths in real time. Our extension of the Vadere simulation framework with interactivity features allows the users to better understand the actual problem situations and to analyze them. Our experiments demonstrate the effectiveness of the approach by simulating the evacuation of students in groups and as individuals from the Schloss Muenster (the administrative building of the University of Muenster) in Germany. During simulation run time, the user can interact with the virtual environment spontaneously (e.g., by opening and closing doors) while our model recalculates the shortest paths for agents in real time.