By a longitudinal account of applications of one Wizard-of-Oz supporting tool, Ozlab, this paper highlights the utility of such tools beyond ordinary design evaluation. The Wizard-of-Oz method does not only allow for performing user test evaluation of interaction designs not yet programmed. Rather, the versatility of a digital but manually controlled prototype allows for many combinations of who determines the mocked up system’s responses and who acts as the prototypical user. It also supports co-design in that users will be presented interactive substantiations of their suggestions as the co-design sessions proceeds.