After an overview of the use of digital shadows in computing science research projects with cultural and social impacts and a focus on recent researches and insights on virtual theaters, this paper introduces a research mixing the manipulation of shadow avatars and the building of a virtual theater setup inspired by traditional shadow theater (or "castelet" in french) in a mixed reality environment. It describes the virtual 3D setup, the nature of the shadow avatars and the issues of directing believable interactions between virtual avatars and physical performers on stage. Two modalities of shadow avatars direction are exposed. Some results of the research are illustrated in two use cases: the development of theatrical creativity in mixed reality through pedagogical workshops; and an artistic achievement in The Shadow performance, after H. C. Andersen.