Abstract : How is "made" the service encounter? To tackle this issue management science has hardly studied, we focus on the "internal" elaboration of the relationship of an organization to its service users. For that purpose, we use the conceptual framework of professional dynamics, defined as the interplay of different collective actors trying to solve a problem. At the occasion of an ethnographic and clinic research within RATP, we precisely observe the evolutions of the treatment of a recurrent problem for public surface transportation: fare evasion. We use two complementary methods to describe the professional dynamics of fare evasion fighting. On the one hand, a historical approach stresses the plasticity of the "fraud" problem and also shows the organizing of its treatment through the 20th century. On the other hand, the analysis of the "Bus Attitude" project, expected to transform the service encounter and the controllers' activity, reveals the opposition between different conceptions of fare evasion fighting. The description underlines the decisive role of the artefacts of fare evasion treatment (especially indicators): they enclose staff and controllers in a repressive understanding of the problem. We call the situation "ontological trapdoor". As an answer to this cognitive constraint, we propose to consider the instrumentalization of the professional dynamics. We retrospectively analyze the progression within RATP of a schematic representation of these dynamics, which was produced for the control staff during the research. Such a use of the conceptual framework favours the actors' reflexivity regarding what they consider to be a "good way" to deal with the fare evasion issue. We finally suggest articulating the description and the representation of professional dynamics, through a collective inquiry. The goal is to enable the actors to pilot the organizing of the service encounter - that is to say to grasp and influence its process of elaboration.