They’re called “guerilla drive-ins,” or mobile movies, because there is no permanent theater. Organizers screen the movies on warehouse walls and in parking lots. And screenings don’t usually take place in the same location twice. Viewers sign up for a service that e-mails them with the time and location a few days ahead of each screening. The movies are generally free, and organizers accept donations to fund the few hundred dollars it takes to secure the location and projector.