In 1978, George Lucas had an idea for a movie called "Raiders of the Lost Ark," and invited Steven Spielberg and screenwriter Lawrence Kasdan to a brainstorming session to flesh it out.
Luckily, the transcript of the session is available online. Here's an inside look at how the character Indiana Jones came to life:
George Lucas: The fact that he is slightly scruffy. You don't know it until it happens. Now, several aspects that we've discussed before: The image of him which is the strongest image is the "Treasure Of Sierra Madre" outfit, which is the khaki pants, he's got the leather jacket, that sort of felt hat, and the pistol and holster with a World War One sort of flap over it. He's going into the jungle carrying his gun. The other thing we've added to him, which may be fun, is a bull whip. That's really his trade mark. That's really what he's good at. He has a pistol, and he's probably very good at that, but at the same time he happens to be very good with a bull whip. It's really more of a hobby than anything else. Maybe he came from Montana, someplace, and he... There are freaks who love bull whips. They just do it all the time. It's a device that hasn't been used in a long time.
Steven Spielberg — You can knock somebody' s belt off and the guys pants fall down.
George Lucas — You can swing over things, you can...there are so many things you can do with it. I thought he carried it rolled up. It's like a Samurai sword. He carries it back there and you don't even notice it. That way it's not in the way or anything. It's just there whenever he wants it.