Summer movies are often described as formulaic. But what few people know is that there is actually a formula--one that lays out, on a page-by-page basis, exactly what should happen when in a screenplay. It's as if a mad scientist has discovered a secret process for making a perfect, or at least perfectly conventional, summer blockbuster.The formula didn't come from a mad scientist. Instead it came from a screenplay guidebook, Save the Cat! The Last Book on Screenwriting You'll Ever Need. In the book, author Blake Snyder, a successful spec screenwriter who became an influential screenplay guru, preaches a variant on the basic three-act structure that has dominated blockbuster filmmaking since the late 1970s.