Interactive systems slowly evolve to become more and more transparent through the merging of physical and digital entities. These advanced forms of interaction are known as augmented reality, tangible interfaces, mixed reality, or Mixed Interactive Systems (MIS). The emergence of these paradigms has a strong impact on the design of interactive systems. Indeed, it leads designers in the handling of more variables and in the exploration a broader design space. To face such a complexity, new formalisms emerged in order to express and describe the fundamentals of these systems. Despite these theoretical advances, in practice most of the design proposals remain limited to the same set of technologies and interaction techniques. In addition, the design methodologies used in traditional HCI do not face this exploration problem. Therefore, to contribute to MIS design, we propose an original approach called "Model Assisted Creativity Session" (MACS), combining a formal interaction model and a collaborative and creative design activity. This new methodology first aims at supporting a more systematic exploration of the design space through the manipulation of concepts expressed in formalism. Then, the objective is to ease collaboration by helping a multidisciplinary team in the construction of language of reference. Overall, the method aims at providing a shared representation between designers and to allow multidisciplinary teams to explore the design space to its most entrenched boundaries thus leading them to reach innovative and creative spaces. To reach this goal, we first identify a set of fundamental principles that describe the boundaries of such a method by analyzing the several paradigms collaborative and creative design and through a review of existing design resources in the field of MIS. Based on this review, we define the core principles of the MACS method. We then investigate to what extent non-MIS experts understand two formalisms of mixed interaction. After that, we describe a set of case studies and analyze the impact of using formalism during a design session through verbal and behavioral protocol analysis. This study shows that the MACS method help participants in focusing on the design problem, in making references to the concepts expressed by the notation for expressing ideas and guide the idea generation. We conclude the presentation of our work by illustrating several MIS designed with this method and by the introduction of an interactive tool aimed at supporting these collaborative and creative design sessions.
