Focused on Grisey’s Les Espaces Acoustiques, the present study is part of a project carried out in collaboration with the conductor Pierre-André Valade and an ensemble of instrumentalists from the Zürich University of the Arts. The documentary collection – consisting of videos of rehearsals, videos of interviews, as well as photos of scores annotated by the conductor – is examined from an analytical perspective in order to provide a better understanding of Grisey’s cycle and its performance. After some theoretical and methodological considerations in relation to the concept of “music as act”, the constitution of the collection itself and the conditions for an analytical approach to it, some avenues of analysis are explored. The first concerns the playing of the horn using a reed, which is examined on the basis of the traces relating to the performative act in a rehearsal context and their relationship with elements of the performers’ discourse. A second avenue concerns the notion of sound fusion, which is studied in a contrasting manner, starting from the performers’ discourse and relating it to the performance activity. This brings to light elements of temporal coordination, control of dynamic profiles, listening strategies, as well as variables of performance, in the context of a major work of spectral music.