There is an undoubted pleasure in watching a classic play unfold to an unknowing audience. I once saw Hedda Gabler in a regional theatre where it was clear from the gasps that most of the audience had no idea that she – spoiler alert – kills herself. But very few people would decide against seeing Three Sisters again because they already know they never get to Moscow – or feel no desire to watch Medea because they are aware she kills her kids.