The term “spitballing,” in its Hollywood usage, connotes a form of free-associative brainstorming. Creative types sit in a room and, through some fizzy, conversational alchemy, produce ideas that the individual participants might never come up with on their own. When it works, it’s an argument against telecommuting. But because improvisation is encouraged, a spitballing session can often sound ridiculous, and the process has been parodied, endlessly, in the movies. My favorite sendup is from “Thank You For Smoking,” when a super-agent (Rob Lowe) proposes to a tobacco lobbyist (Aaron Eckhart) that they work out a product-placement deal for a science-fiction movie.

Eckhart: Cigarettes in space… But wouldn’t they blow up in an all-oxygen environment? Lowe: Probably. But it’s an easy fix. One line of dialogue: “Thank god we invented the, you know, whatever… device.”

But while this kind of satire is a mainstay of movies about the movies, we seldom get to witness the actual creative sessions that go into producing films. So I was gobsmacked to discover, recently, that over several days in 1978, George Lucas, Steven Spielberg, and the screenwriter Lawrence Kasdan worked through an idea Lucas had for a film called “Raiders of the Lost Ark,” and they recorded the sessions. And there’s a transcript. And it’s online.

After “Star Wars” opened, in 1977, Lucas had sought refuge in Hawaii, and it was on the beach there that he first pitched Spielberg the idea for a film that would recapture the rollicking adventures and Orientalist locales of the matinee serials of Lucas’s youth. Spielberg, who had already enjoyed huge success with “Jaws,” was interested. Having enlisted Kasdan to write the script, they assembled in a little house in Sherman Oaks that belonged to Lucas’s assistant, and, over several days, concocted the film.

Lucas walked into the meeting with an outline of the story, but he wanted to flesh it out with his writer and director. In the transcript, he begins by articulating a recipe for the contemporary blockbuster: the picture will consist of one big set piece after another.

“And each cliffhanger is better than the one before,” Spielberg adds, warming to the idea. “What we’re doing here, really, is designing a ride at Disneyland.”

The hero, Lucas explains, is a globe-trotting archaeologist, “a bounty hunter of antiquities.” He’s a professor, a Ph.D.—“People call him doctor.” But he’s a little “rough and tumble.” As the men hash out the Jones iconography, they refer, incessantly, to other films, invoking Eastwood, Bond, and Mifune. He will dress like Bogart in “The Treasure of the Sierra Madre,” Lucas says: “the khaki pants…the leather jacket. That sort of felt hat.” Oh, and also? “A bullwhip.” He’ll carry it “rolled up,” Lucas continues. “Like a snake that’s coiled up behind him.”

“I like that,” Spielberg says. “The doctor with the bullwhip.”

After establishing his hero, Lucas proceeds to walk through the film’s plot, beat by beat. There’s the opening sequence in South America, which Lucas describes as “misty and primeval, ‘King Kong’-ish.” (If you want a further sense of what a clever pastiche of earlier films “Raiders” ended up becoming, and haven’t seen this astounding supercut, it is well worth watching.)

Lucas captures the second act of the film pretty aptly:

In the essence it’s just bullshit stuff where he wanders around Cairo trying to uncover the mystery of his puzzle. At the same time, you meet all these interesting characters and every once in a while somebody throws a knife at him, or he beats somebody up, or somebody beats him up. Typical Middle Eastern stuff.

But most importantly, the film had to hurtle at a furious clip. Lucas envisioned the whole story as one elaborate chase: the hero chases Nazis, Nazis chase the hero, and everyone races to find the Ark of the Covenant. They needed a love interest, of course. “She’s sort of a Marlene Dietrich tavern-singer spy,” Lucas suggests, of the character who would become Marion Ravenwood (Karen Allen). She works in a bar, he continues. It’s like Rick’s Place, in “Casablanca.”

Kasdan: This is in Cairo? Lucas: No. This is in Nepal. She’s stuck there. Kasdan: Who are her customers at this Rick’s Place in Nepal?

The filmmakers want Marion to have a romantic history with the hero. They also want to cast a young actress in her twenties, however. This raises logistical questions. But it’s an easy fix:

Lucas: He could have known this little girl when she was just a kid. Had an affair with her when she was eleven. Kasdan: And he was forty-two. Lucas: He hasn’t seen her in twelve years. Now she’s twenty-two. It’s a real strange relationship. Spielberg: She had better be older.

Over the intervening decades of enormous wealth and success, both Lucas and Spielberg have carefully tended their public images, so there is a voyeuristic thrill to seeing them converse in so unguarded a manner. 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.