One important tool for developing complex interactive applications is "Wizard of Oz" (WOz) simulation. WOz simulation allows design concepts, content and partially completed applications to be tested on users without the need to first create a completely working system. In this paper we discuss the integration of wizard interface tools into a Mixed Reality (MR) design environment and show how easier creation and evolution of wizard interfaces can lead to an expanded role for WOz-based testing during the design evolution of MR experiences. We share our experiences designing an audio experience in an historic site, and illustrate the evolution of the wizard interfaces alongside the user experience. KEYWORDS Wizard of Oz, design tools, authoring, prototyping, mixed reality, ubiquitous computing, experience design ACM CLASSIFICATION KEYWORDS H5.2. Information interfaces and presentation (e.g., HCI): User Interfaces - User-centered Design, Prototyping