When they were coming up with the details for Raiders of the Lost Ark, George Lucas, Steven Spielberg, and writer Lawrence Kasdan recorded their spitballing sessions, and a transcript (pdf) is now available online. Patrick Radden Kaffe is fascinated:
As the screenwriters Craig Mazin and John August pointed out recently on the Scriptnotes podcast, one delight of reading the transcript is watching Spielberg throw out bad ideas, and then noting how Lucas gently shuts him down. Spielberg, who had sought to direct a Bond movie—and, astonishingly, been rejected—thought that their hero should be an avid gambler. Lucas replied that perhaps they shouldn’t overload him with attributes. (Lucas himself had briefly entertained, then mercifully set aside, the notion that his archaeologist might also be a practitioner of kung fu.) There’s a good reason we seldom get to spy on these conversations: really good spitballing, like improv comedy, requires a high degree of social disinhibition. So the writers’ room, like a therapist’s office, must remain inviolable.
Spielberg fires off ideas with an adolescent’s stamina—and not all of them are bad, either. In fact, among his spontaneous interjections are some of the most iconic episodes in the film. “I have a great idea!” he exclaims. “There is a sixty-five-foot boulder, that’s form-fitted to only roll down the corridor, coming right at him. And it’s a race. He gets to outrun the boulder!”
The Dish 