Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
This is based on two short novels by John Steinbeck - Cannery Row, set in the 1930s, and its sequel Sweet Thursday, which takes place in the post-war 1940s, as does this movie. The movie is mostly concerned with Doc, his neighbors the Bums, the Bear Flag brothel, and Suzy and her romantic relationship with Doc. Unfortunately, the movie leaves out my favorite character in Cannery Row - the gopher - who is worthy of his own novel.
The rattlesnake scenes are based on events in Sweet Thursday. Doc, played by Nick Nolte, is a biologist who lives on Cannery Row in Monterey, who collects animals and sells them. Suzy, played by Debra Winger, is a vagabond who recently came to town so desperate for work that she took a job at the local brothel. They are immediately attracted to each other but they keep fighting each other off. When Suzy's matchmaking madam sends her to deliver some cookies to Doc, they interact for the first time in a typical meet cute scene. Suzy walks in and puts the bowl of cookies on top of a box that she doesn't realize is a rattlesnake cage. The snakes rattle loudly startling her. She asks Doc why he has the snakes and he tells her that he sells the venom. She offends Doc by telling him she wouldn't want to live with a bunch of filthy snakes. He fires back, telling her that snakes are cleaner than humans - they even shed their skin, unlike humans. It's not a great start to a relationship, but we can see there's a lot of chemistry - mostly the explosive kind.
Much later in the movie, after a few more failed attempts at romancing Suzy, Doc is at home putting on a tie and his good clothes to visit Suzy after she has moved out of the brothel. He wonders if he is doing the right thing, dressing up and bringing chocolates to her, so he talks to the rattlesnakes and asks for their opinion. They're silent, so he bangs on the glass to get them to rattle a response, which he assumes means he's doing the right thing.
We see a live Southwestern Speckled Rattlesnake in a cage and some other live rattlesnakes, but the snakes Doc talks to are stuffed dead snakes, or fakes, and when he bangs on the glass and we hear them rattle, their rattles do not move.
There is also an extensive use of frogs in the movie.