Testing predictions on the use of graphical interfaces can result on redefinition of systems, and often changes the main assumptions of interface programmers. This work presents experiments done with a graphical geometric modeler. The system's interface is heavily based on direct manipulation of tridimensional entities; many tasks are guided by a moving plane, also controlled by direct manipulation. The events observed during the tests with users are classified. Based on the results and events observed, a methodology is proposed and used to redefine the system's interface.