The MTV Movie Blog has been kicking out with the Kingdom of the Crystal Skull writer, David Koepp:

As for the theme of the movie and how it fits in with the existing trilogy,

“I hope it’s along the lines of the first one,” he said. “It’s certainly not as dark as the second one and it’s certainly not as humor driven as the third one. Hopefully it will have its own place [but] it’s closest to [’Raiders’].”