Barbara McNamara says that her job as a background casting director is to turn the pages of a script into the real world. She often hires hundreds of actors every day to set the scene in a crowded coffee shop, a convention center, or the bustling New York City streets. Yet casting extras for the NBC comedy “30 Rock,” which she has done since the show’s first season in 2006, presents a fun and unique challenge for the CD because its zany characters so often redefine reality. “It is way out there sometimes,” she says of the show. “You name it, and I’ve had to cast it, from subway musicians to walking Statues of Liberty.”