This contribution refers to a companion modelling approach applied to the study of interactions between social and environmental dynamics in a small protected island, part of a biosphere reserve. This approach leads to a role-playing game designed by a pluridisciplinary workgroup (researchers and reserve manager), used as management support and as a tool to help people regarding the stakes of sustainable development. For several years, Ushant Island (Brittany, France) was in the process of social and environmental restructuration due to land use and land cover changes. Major land use changes resulted in fallow land encroachment and its consequences on landscape, traditional activities and biodiversity. This situation raises the question as to the different types of actors (stakeholders, scientists, local population) in relation to the future and the consequences of individual actions on common good. The stakes of sustainable development are set. In this study, the multidisciplinary process leads to a geographical agent-based model, simulations and a role-playing game. The role-playing game, final step of our companion modelling approach, is developed in order to assist the awareness raising and the involvement of all, be they elected representatives or not. This contribution describes the steps which led to the role-playing game design, the contents of the play sessions, their results and prospects in terms of learning tools. It examines the place of companion modelling and associated tools in the context of a dialogue, whose main issue remains the following question: "How?"