George Lucas and Steven Spielberg had dreams, some that they shared, and when they reached the point where they both had Hollywood clout, they could so any kind of film they wanted. So they designed a thrill ride that gave us Indiana Jones.
“What we’re doing here, really, is designing a ride at Disneyland,” Spielberg kept telling his collaborators. Spielberg wanted Raiders to be less of a linear story and more of a series of increasingly giddy cliffhangers. Spielberg, Lucas, and Kasdan designed Indiana Jones to be a classical, mythic man of action, the kind of guy who steals the horse and launches himself after the truck without thinking twice about it. They succeeded wildly. Raving about Raiders, Roger Ebert wrote, “It’s actually more than a movie; it’s a catalog of adventure.”
They actually designed the action scenes first, then filled in a plot just to string those scenes together. Read how it happened at The A.V. Club.