This thesis introduces the research of combining the field of interaction design with the domain of the classical concert. The research is framed around a curiosity about why interaction at big stages tend to fail: how can mass interaction support the concert experience in a way that interactivity becomes a dialogue between artistic intention and audience experience. The work is centered around a collaboration with The Royal Danish Theatre and The Royal Danish Academy of Fine Art – The School of Design, focused on a scheduled classical concert at The Royal Danish Opera, where a concept is tested. The work is carried out as an explorative work of a design space through interweaved processes of design practice and reflection, emphasizing the need to include the artistic intention and to support the experience of the performance. Validation of the work is triangulated: empirical results from interviews and observation at the concert, supported by theoretical aspects, related work, reflection, and analysis. Additionally validation is drawn from the collaboration, as well as on a micro level: all engagements are part of the validation, making reflections in iterations and through material exploration. With a focus on the whole, both regarding methodological points of view as well as the specific ideal to include artists and artistic intention, the work adds a new layer to the HCI (Human Computer Interaction) tradition otherwise dominated by a focus on the user. In conclusion, the work brings forth four provisional takeaways to the design space of mass interaction: Reserve interaction for dramaturgical significant moments, Breaking norms creates social playfulness and disruptive behavior, Create tight coupling between action and meaning, and Tie the stage to the whole space. The specific concept at the concert will be part of a repertoire and possibly be an inspiration to the design community and cultural institutions.