In Peter Gelb's office at the Metropolitan Opera, New York, there's a screen that's flush with the wall so it resembles a window. It captures whatever is happening on the Met's stage – so its general manager's eye can be trained on rehearsals and performances all day long. When I visit, the set of Philip Glass's Satyagraha is being taken down, to be replaced, a little later, by that of Don Giovanni...

Photo by Dario Acosta