Since the end of the eighties, agent-based simulation has demonstrated its usefulness for the modelling of complex systems. However, a difficulty raised by these simulations concerns the designing of agent behaviour. Indeed, most of time, it is difficult for experts to translate their knowledge in a formal way that can be directly used by a computer. In order to facilitate the definition of such behaviour, we propose an approach based on a participatory method. In our approach, an expert directly plays the role of an agent in the simulation. Then, the expert behaviour is analysed in order to extract formalised knowledge about the agent behaviour. In this paper, we propose to formalise the agent behaviour by using a combination of production rules and of a multi-criteria decision making method. An experiment, carried out in the domain of ecological simulation, is presented. This first experiment shows promising results for our approach.