“I’m very much a beginner,” the actress Britt Lower said. This was on a mid-December evening at a warehouse space in the Williamsburg neighborhood of Brooklyn. Dressed in a T-shirt and houndstooth-patterned leggings, Lower climbed a ladder to a narrow platform where an instructor clipped her into a safety harness. Seconds later, Lower was upside-down, swinging from the ceiling of the España-Streb Trapeze Academy. She dismounted, exhilarated. It was her fifth trapeze class.