What happens when you’re working on the Emmy-winning show The Marvelous Mrs. Maisel? Well, you get to see big-name actors being regular people. You get a 1950s-style hairdo and costume. And, you take a quick nap leaning up against a fence between early-morning takes. At least, that’s what it was like for Meryl Prendergast, a second-year theater student at Northeastern who was cast as an extra for the show’s second-season finale.