Studies of composers’ cognition are restricted by the methodological and epistemological problems with which they are associated. In order to capture the cognitive processes involved in the elaboration of an instrumental piece by Stefano Gervasoni entitled Gramigna (2009-), the authors developed different strategies for composition simulations, which were filmed in order to collect rich and structured data. These methods draw upon the precompositional documents established by the composer over the course of his creative activity, by requesting that he re-enact the operations that generated the documents and the final work. The article focuses on the first movement of Gramigna, composed in a single day, several months before this study began. During a “compositional situation simulation interview”, the composer sat before his manuscript in order to induce his recollection, simulation and verbalisation of the completed creative act. The study’s methodological and epistemological principles (including their advantages and limits) are presented first, followed by a detailed reconstruction of the composer’s action/ cognition during the writing of the manuscript. This procedure highlights the sequence of Gervasoni’s significant compositional operations and allows us to categorise and formalise the most salient features of his compositional process. These include the creation of locally applicable construction rules, the placement of concurrent rules, the attention paid to irregularities in the formalistic systems employed and tactics that blur notions of referentiality.