Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
These comments are going to spoil the movie, but I just can't avoid it, so beware.
Somewhere back in the days of the wild west, Kirk Douglas hides two bags containing a half million dollars of stolen money in a "den" or "nest" full of rattlesnakes inside some rocks in the mountains. (Because movie rattlesnakes always hang out together in groups unlike real rattlesnakes which sometimes do that but only in the winter.) When he goes back months later to get the money, first he shoots all the snakes, then, just in case one survived, he wraps his arm up with his coat and puts on a leather glove before reaching down to pull out the bags. Smart guy. Or is he? He opens the saddlebag and handles the money with a big smile. When he opens the second bag, made out of a woman's bloomers, we hear a loud rattling sound effect and see a rattlesnake jump out and bite him on the neck.
They used several real rattlesnakes in the shot of the den. When he shoots them we see small explosions, but no snakes are hit. They cut away then cut back to the den and it is full of dead snakes. These look like real dead snakes. Maybe they rounded up some roadkill, or maybe they killed the rattlesnakes. Some kind of fake snake is pushed or pulled out of the bag into Douglas' face. Then they cut away to a shot of a Western Diamond-backed Rattlesnake crawling away. A comment on IMDB states "Twelve rattlesnakes were used for a key scene but failed to hiss on cue. The hissing had to be dubbed in during post-production." In the film we hear rattling sounds, not hissing. They should have hired snakes that are union extras that can rattle on cue. What were they thinking?