In the 1981 film Raiders of the Lost Ark, one particular scene consistently brings the house down -- Indiana Jones, having survived an elaborate chase through the casbah, is confronted by a swordsman whipping through a flashy routine with a scimitar. With a look of infinite fatigue and disgust, Indy simply pulls out his gun and blows the bad guy away.
That bit flowed not from the pen of a screenwriter but from the desperation of Harrison Ford. His desire to spend less time on this scene and more in a washroom led to an actor-inspired script change that was ultimately worked into the film.
Three months' of shooting in the blazing heat in Tunisia had resulted in a terrible bout of dysentery for Harrison Ford. The original scene, as called for in the script, would have required approximately three days to shoot. Not looking forward to another lengthy shoot under that blistering sun, Ford persuaded Spielberg to try the scene this much shorter way. (One could say Ford was given "the runs" of the place.)
Occasionally, some confusion surfaces as to who -- Ford or Spielberg -- had the notion for this script change. The principals themselves don't appear to disagree on it being Ford's baby, but those who've heard the diarrhea story second- and third-hand have been known to get muddled on this point. (In a 1995 interview, George Lucas attributed the creative spark to Steven Spielberg, for example.)
As for whose idea it was, according to a 1981 interview with Steven Spielberg:
Ford developed dysentery in the blistering 130-degree heat of Tunisia, where the cast and crew had to fan their mouths constantly to keep out flies looking for shade (one crawled into chief villain Paul Freeman's mouth during a crucial scene). Too weak to swing his whip, Indy was slated for a 3 1/2-page fight when Ford had a better idea. "We had Indy pull out his revolver and dispatch the dude," says Spielberg of the film's funniest scene.