One hundred years ago, the hills above Los Angeles got a facelift. A giant sign was erected to advertise a new real-estate development. Its 13 letters, each 43 feet tall, spelled “HOLLYWOODLAND” (“land” was later dropped). The modern movie business was forming around the same time as Warner Brothers consolidated power and Walt Disney left Kansas City for Los Angeles. Yet instead of celebrating its centenary, Hollywood is preparing for upheaval: screenwriters are poised to strike for the first time in 15 years.