Greetings from the Overlook Hotel.
Speaking of the Well of Souls, that place was plenty spooky well before it was filled with snakes. The tomb was constructed on the same British Elstreet Studios stage as the "Colorado Lounge" set built as the foyer of the Overlook Hotel in The Shining.
I hate snakes.
The production grabbed all of the snakes they could find in the pet shops of London, which explains why there are so many different types of snakes native to different parts of the world. But there simply were not enough snakes. So legless reptiles were added into the mix along with several pieces of chopped up hoses. The movie's first A.D. was bit by a python during filming. Indy's terrifyingly close staring contest with a cobra was filmed with a nearly invisible piece of glass between them. Those cobras were of course super dangerous - one of them bit and killed a python.
Unlike the snakes, there were plenty of tarantulas on-hand to do the job for the scene where Alfred Molina's back is covered in them. Unfortunately, they were just standing still. Spielberg wanted them to be crawling, so the animal wranglers inserted a single female into the otherwise all male pack of crawlers, sending them into a tizzy. Of course it wasn't the actor's final brush with arachnids. Decades later, Molina would play Otto "Dr. Octopus" Octavius, the primary villain of Spider-Man 2.
That monkey was just hungry.
It also took some animal wrangler ingenuity to get a monkey to cooperate. After several unsuccessful attempts to get the monkey to throw a comically evil Nazi salute, a fishing pole dangling a grape just out of frame did the job. Sorry, monkey. We know you didn't mean it!
Just shoot the sucker.
Several members of the Raiders of the Lost Ark production became ill with fever and food poisoning during the hot and arduous shooting days in Tunisia. In the script, Indy was supposed to use his whip to disarm the expert swordsman who flashily challenges him to a duel. After several failed attempts to get it right, Ford asked the director, "Can't I just shoot the sucker?" And it became a massively beloved scene.