Ask your assistant to go and stand with her back to a wooden frame and strap her in with a few heavy-duty, leather straps. Padlocks keep the straps firmly in place. An awkward predicament for your assistant. Next, close the big curtain which is suspended, in a circle, round the frame. Because you have to go right round the frame, you will, halfway round, be hidden, momentarily, from the spectators' field of vision.
Their surprise will therefore be total when they suddenly see your assistant emerge from behind the frame, curtain in hand, to complete the circle. In the same flourish, she draws back the curtain. Whoops of approval from the audience as they see you captive on your own frame!
An unexpected, highly-entertaining and lightning switch. Very easy to perform.