Paradoxically, some ballet masterworks absolutely depend on tiptop performing to demonstrate how great they are. If they don’t get it, they can look like the dodgiest of curiosities — did people in those days really rate this stuff? A whole genre of fiercely zipped tragedies of feeling emerged in Britain and the US in the 1930s and 1940s, fed by Antony Tudor, Frederick Ashton and Robert Helpmann over here, and Martha Graham and José Limón over there; works that, unlike classical dance, require acting skills of rare force and perfect pitch.