Perhaps Bob has been telling Sally for weeks that the wedding pictures should come down to make way for a painting he's bought. Maybe Sally doesn't usually make chocolate cake but put it there now to help ease the pain of her going. All of these choices help to create a more layered and meaningful feel to the screenplay. This page has an assignment , which you must complete to receive a final grade. Deciding when and how to use these devices is a writer's dilemma.
Some writers map it all out in advance.
Others find things as they move along. But no writer ever attempts to create meaning without having a full and true knowledge of character. If all story flows out of character, then the personal history and behaviour of the characters is essential to finding layering and meaning. Bob's marriage is on the rocks. Sally has made a dramatic choice, but has also tried to soften it.