This paper focuses on innovation, understood as a critical step of the design process: the implementation of an artefact, and its re-design in work practices. Based on a model of "appropriation" (presented in the article), the article focuses on exchanges between designers and workers, interacting around multiple outlines of the object during its design. What develops at the border, between different communities interacting one on the other, and to what extent can an interventionist contribute to this process? An interpretation in terms of mutual trainings between heterogeneous "professional worlds" is proposed, to articulate through the development of a "common world." From one case, it is proposed to distinguish three levels for grasping the development of activities.