We introduce the Blocking Notation as a minimal formal system to describe a complete theater performance, and we make recommendations for a user-friendly interface dedicated to notating performances based on video recordings. This interface should give stage directors the opportunity to visualize the play at the earliest stages. It should also enable stage managers to easily take notes in order to make sure that every aspect of the production runs just as the director intended time after time. Preliminary experiments demonstrate the usefulness of the notation on an experimental data set of three short scenes in four different productions of the Marivaux play, "The Island of slaves".