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 comedy/mystery that tries hard to be like an Alfred Hitchcock thriller. Goldie Hawn is a librarian and Chevy Chase is a police detective who falls for her and the setting is a beautiful late 1970s San Francisco. A man Hawn picks up hitchiking slips a pack of Marlboro cigarettes with a roll of film in it into her purse. Bad guys kill him trying to get the film, then kill others and chase and kidnap Hawn. But ultimately the film is just a McGuffin.
The snake shows up just after Hawn has witnessed a murder and runs home. Her landlord, Burgess Meredith invites her to his apartment seeing that she is upset. As they talk in the living room we see a boa constrictor in the background slowly crawling on the floor towards them with tense music in the background. Meredith is telling stories of being in the field in Africa with witchdoctors, and this increases our uneasyness about the snake. We are being led to think that the snake is dangerous and maybe even be part of the murder plot Hawn has become involved with. The snake slowly crawls next to Hawn's feet, up a coffee table and over a plate of cookies, and when Hawn finally sees it she makes a startled noise with a surprised look on her face. But it turns out it's not the usual panic and fear in her snake face, just surprise. Meredith yells "Esme, get away from those cookies" then cruelly smacks the snake several times, as if the snake was trying to eat them, as we learn the snake is his pet named Esme which he lets crawl around his apartment. Hawn picks and cuddles the snake up and tells Meredith that he frightened her. He responds: "She's getting too uppity. She steals me out of house and home" as if a snake would eat human food. Maybe we're supposed to know how absurd that is and find it funny, it's hard to know.
We see the snake again in Hawn's apartment, as it is crawling towards the pack of cigarettes everybody is looking for that is partly hidden under a plant. Meredith grabs the pack of cigarettes and tells the snake not to eat them, then he throws them into a fireplace and we can see by the dark smoke that the film is burning. The snake then opens its mouth and we hear a raspy hissing sound that is broken up to make it sound like the snake is chuckling. The close-captioning confirms this. This is a comedy, and comedies exist in their own universe, so I can't criticize it for its unrealistic depiction of a snake that eats cookies and cigarettes and laughs for no apparent reason.