SHEFFIELD, England — Visitors to Tudor Square in the center of this northern English city might spot some unusual figures there this week: a woman sprinting through in a neon boilersuit, or a tutu, or a man running with a box of scissors. And if they look like they’re in a hurry to get somewhere, that’s because they are. These are actors, and they have an entrance to make — on a different stage from the one they just left.