Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
Tim Robbins, a studio executive who has been getting threatening postcards from a disgruntled writer. In a nice twist on the Rattlesnake-In-A-Mailbox method of killing someone (which was really used against lawyer Paul Morantz) Robbins gets a post card in his mailbox showing a rattlesnake (a Red Diamond Rattlesnake) from someone he was supposed to meet in a bar. When he gets to his SUV and starts driving, he gets a car fax telling him to look under his raincoat. Underneath he finds a fancy box with a post card with the note "do not open 'til Xmas!" When he lifts the lid there is a rattling Red-diamond Rattlesnake inside the box. (Movie sound design rule number 61: If you see a snake, you have to hear either rattling or hissing.) The snake crawls into the SUV, so Robbins stops the truck, tears his shirt off action hero style, and whacks the snake over and over again with an umbrella. Then he pulls it out using the umbrella handle as a snake hook. It looks like the snake here has been replaced with a lighter colored snake but it could just be the lighting.
Later when Robbins is driving to Desert Hot Springs, we see another Red Diamond Rattlesnake curled up on a rock above the road, rattling, of course. That's what they do.
Kudos to Robert Altman, the director, for his consistancy in using the right species of snake for the area and for not roughing it up too much. That's probably the main reason this film is regarded as one of his best in a long line of excellent films. Yep, that must be why.