It's the Little Things
When Walter Neff, insurance salesman (Fred MacMurray), overtly flirts with Mrs. Phyllis Dietrichson (Barbara Stanwyck), mentioning her prominently-displayed anklet, Phyllis tucks one ankle behind another in a gesture of sudden modesty.
Later in the scene, we almost hear the gears turning in her head as Phyllis forms the idea to kill off her husband and collect the insurance money using Neff's connections. She then crosses her legs so that the anklet is displayed again. This is her first deliberate lure.
She's still keeping herself aloof at this point, controlling this newly-forming relationship, but Phyllis is definitely on the hunt in one little gesture. She's reminiscent of an anglerfish - dangling something shiny and alluring in front of her victim.
Walter - who flirts without caution as if this is his regular behavior on a sales call- thinks it's just a lonely, discontented wife looking for a promising partner. He has no idea what he's getting into.