[Moonman62]

Truly, one of my favorite scenes in movie history.

...

I read that it was mostly improvised due to Harrison Ford being ill.

[Publius]

True. Ford has stomach flu, and his attempts to disarm the swordsman with the bullwhip failed after many takes. In frustration, Ford used the blank pistol, the swordsman went with the flow -- and the cast and crew were laughing so hard that it took almost ten minutes for Spielberg to restore order.

When Spielberg looked at he rushes at the end of the day, he discovered that what he had shot was better than what had been written in the script by Katz and Huyck. He went with it for the final cut.

He did the same thing when the actor who played Bellocq swallowed a fly on one take.