Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
This is a fairly mediocre crime drama with some interesting twists in an unusual location - the Arizona desert. Debra Paget, Anthony Quinn, and Ray Milland are all on the run in Arizona trying to cross the Mexican border. They stop in a cave on a rainy night and Quinn and Milland start to fight over a gun. The gun goes flying over to one side of the cave and both men go for it, freezing when they see that it landed next to a rattlesnake. Quinn tells Paget to shoot it with a rifle. She worries she'll hit one of them instead but finally shoots the snake.
We see some stock footage of a live black rattlesnake, maybe an Arizona Black Rattlesnake or it could be a Mexican rattlesnake species I don't recognize since the movie was filmed in Mexico. The snake we see in the fake studio cave with the actors is a silly-looking rubber snake with its tail being held up and shaken. When the snake is shot, they just pulled its head up by a wire so that it flies up into the air. And for some reason we see the dead snake still quivering and hear it rattling behind Milland for a couple of minutes.
The snake scene didn't advance the plot or the characters much other than showing us that Paget could shoot and was in the movie to be more than just the bubblebath-taking cheesecake we've already seen. It was probably just thrown in to inject some extra excitement into a movie that was getting a bit slow.