The concept of "milieu" emphases the complex business network in which project marketing takes place. In particular, it allows better identification of relevant inter actor relationships in both terms of time and space all along project phases. We look at this concept of "milieu" again to precisely question the interactions between project marketing and project management during such phases. This interrogation is based on a concrete case of relation BtoB in industrial area (automotive industry). In particular it proposes logics, respectively constructivist and determinist, of the main protagonists (company of engineering manufacturer). The object of this article is to highlight the complexity of dynamics of actor within the various phases of the project: we will notably underline the difference and the accommodation between limits of interaction and the dynamics of congruence.