Famously, the original Raiders of the Lost Ark swordsman fight scene was intended to be an elaborate whip vs. sword brawl between Indiana Jones and his adversary. However, what's given in Steven Spielberg's film, also the first in the Indiana Jones series, is comically far from elaborate. It's a well-known movie moment, arguably the most defining one of Indiana Jones' adventures: the heroic archeologist is being chased through Cairo, Egypt by Nazi-aligned mercenaries, when he's suddenly confronted by a menacingly brave swordsman. Brandishing his weapon with impressive skill and bravado, the swordsman confronts Indy with what's shaping up to be the most epic fight scene of all time, only for Indy to shoot him, effectively ending the would-be battle.
Of course, this scene, as everyone now knows in the internet age, was improvised. The gun is not in the script, which instead envisions Indiana Jones using his whip in tense battle. The decision to switch to a gun was made on the fly, partly because Harrison Ford was sick with dysentery during filming that part of the movie. Anyone familiar with dysentery can understand why the change was made: performing the stunt in the hot desert sun for a sequence that would require multiple days of shooting isn't exactly ideal for someone needing to run to the bathroom every five minutes. And so, smartly, Ford suggested the swap to a gun, which Spielberg thought better represented Indiana Jones' character, and the rest is history.
But, again, because this is the internet age, there is an alternate history available. Over the years, some footage of the originally intended fight sequence has been revealed. In it, the initial direction of Indy using a whip substitutes the calm, cerebral, Clint Eastwood-like coolness of the final product with a physical demonstration of Indy's "whip smart" inventiveness. Yet, knowing the gastrointestinal distress he's working under, one can't watch the deleted scenes without feeling some sympathy for Ford.
All things considered, despite the cinematic excitement of an all-out whip vs. sword duel, it's largely agreed that the improvisational decision worked better for the character of Indiana Jones. If anything, what makes the scene so memorable is the authenticity of Ford's acting—the way he tiredly wipes his brow, the genuine annoyance with the swordsman, the "onto the next thing" look on his face when he's leaving the fight.
No doubt, after months of training for the role, Terry Richards, the British stuntman who played Indy's adversary, likely felt the final product was drastically anticlimactic. Yet, despite Richards working on over 100 film and TV productions from the '60s through the '90s, including the James Bond, Stars Wars, and Rambo franchises, his perhaps least remarkable action scene turned out to be his most successful, career-wise, all thanks to Indiana Jones' weak stomach.