Purpose – The purpose of this paper is to focus on the professional activity development of prop makers. These professionals are responsible for creating a huge variety of objects for the stage, ranging from furniture and soft furnishings to weapons, statues, jewelry and animated models. A feature of the work is to create objects that are new almost every time. In Western Switzerland, there is neither initial training nor continuing education for the profession of prop maker. Therefore the aim of this study is to better understand the professional practices and informal learning of prop makers at the Grand Théâtre de Genève. Design/methodology/approach – Given their interest in the details of how work is learned and carried out, the authors used ethnographic methods to study the prop makers' working practices. These methods place an emphasis on the detailed observation of practices through intensive, long-term involvement. The fieldwork began in November 2005 and ended in May 2007. This period was organized into three phases articulating direct observation and participant observation. The data were processed according to the methodology of grounded theory. Theoretical sensitivity came from a number of sources; however, French-speaking ergonomics, and particularly the course-of-action theoretical framework, have largely determined our conception of activity and workplace learning. Findings – The results allowed the authors to identify the core of a substantive theory of prop makers' activity and self-construction. Three components formed the core of this theory: conservation, invention, and distribution (CID). These three components are essential to understanding how prop makers are able to achieve, maintain and develop professional expertise both individually and collectively in the near total absence of initial and ongoing training and in a context of constant demand for high technical performance. Research limitations/implications – Despite the limitations of this study and the need for caution, the study seems to have two main implications. First, it leads to the reaxamination of the concept of informal learning and to assume the self-constructive dimension of activity. Second, it encourages studies to question the triple developmental process: technical, individual, and collective. Further studies are needed to better understand the triple process of individuation (technical, individual and collective) that operates in work situations and to test the heuristic power of this notion to account for learning and development in the workplace. Originality/value – The originality of this work is to address the issue of professional development in relation to the work of Simondon Gilbert on technical invention and his theory of individuation.