George Lucas, Steven Spielberg, and Lawrence Kasdan brainstorm ideas for an upcoming project, Raiders of the Lost Ark. Proof that ideas are easy; it's the execution that's hard.
Trying to come up with scenes for a 30's style action serial, it's amazing how quickly they're able to hammer down all of the big scenes in Raiders as well as ideas that show up in Temple of Doom and the Last Crusade.
Lucas is good with concepts but tends to overstuff every scene like an over-excited kid. Spielberg's ideas tend to sound goofy, but he's good with character moments. Kasdan seems to hold the whole thing together while coming up with good spins.