The theme of a work is distinct from its subject, which is what the story is ostensibly "about." The theme is an expression of the writer's views on that subject. The subject of a story, for example, might be war, while the theme might be that war occurs in endless cycles that humanity is forced to repeat.
Writers can present a literary theme in many ways. The thoughts and actions of different characters may serve to express the theme, whether explicitly or subtly. The theme might also be expressed through conversations and dialogue between characters. Different characters can often represent different aspects of a theme. In Dostoevsky's "The Brothers Karamazov," for example, brothers Ivan and Alyosha represent the positions of skepticism and religious faith, respectively, and together represent the novel's theme of the conflict between the two. In the greatest works of literature, themes are carefully intertwined with all the other elements of the story and are only fully revealed as the text progresses.