To best develop the theme of a story, character motivation and plot are the most important details. Character motivation drives the actions and decisions of the characters, revealing their desires, conflicts, and growth throughout the story. This, combined with the plot, which outlines the events and structure of the narrative, helps to illustrate the central themes and messages the author intends to convey.
While readers' prior knowledge and the author's motivations for writing can influence interpretation, and descriptions of time and place can provide necessary context, it is the interplay of character motivation and plot that most directly shapes and reveals the theme.