We’re all talking over coffee at the Soho Hotel during a break in rehearsals: co-founders Bond, Lloyd and Leighton, a more recent recruit. If it sounds rather stressful putting all this on, well, it is. The show relies on 15,000 volunteers to build the sets, source the props, act out the scenes — and there is no guarantee anyone is going to show up, let alone do what they’re asked to do.