In the schlocky world of '50s/'60s horror film marketing, William Castle was the undisputed king, and the "Emergo" effect was possibly his greatest promotional gimmick.
Castle was a producer and director with a live theater background and a reputation for being able to crank out competent, crowd-pleasing B-movies on time and under budget. He also possessed a showman's shrewd understanding of what it took to fill cinemas in an age when television was slowly overtaking the movie-going experience.
In 1958 he had the idea for a live-action special effect he called "Emergo", and so he produced a movie to justify that effect, telling the screenwriter that he didn't care what the story was about as long as it featured a skeleton emerging from a vat of acid.
The resulting film was titled House On Haunted Hill and starred Vincent Price as an eccentric millionaire who invites guests to stay in his haunted mansion. In true proto-Scooby Doo style, it is eventually revealed that the millionaire was faking the haunt in self-defense against a murder plot. None of that mattered very much to Castle's first matinee audiences, who were on the edge of their seats waiting to experience Emergo.
At the crucial moment, as the apparently ghostly skeleton appeared on-screen, a glowing plastic skeleton would emerge from behind the screen and "float" out over the cinema audience via a system of wires and pulleys. Meanwhile, Vincent Price's on-screen character was seen manipulating a gadget that appeared to be controlling the flying specter, maneuvering it until it returned to its lair.
The novelty of Emergo caught the public imagination and, as word slowly spread from town to town, local kids would compete to see who could be the first to hit the flying skeleton with thrown popcorn. The important thing to Castle, of course, was that they had paid to get in.
Castle's later promotional gimmicks included "Percepto" (electric buzzers installed in cinema seats, for The Tingler) and the "Punishment Poll" (in which audiences were purportedly able to "vote" for the punishment of the on-screen villain Mr. Sardonicus). They also inspired the underrated movie Matinee (1993), which starred John Goodman as the very Castlesque "Lawrence Woolsey".
In recent years, some small cinemas have begun staging William Castle homages (especially around Halloween), offering modern audiences the same tongue-in-cheek thrills that were his stock-in-trade sixty years ago. Of course, no modern screening of House on Haunted Hill is complete without its own take on Emergo: