Early in the first part, McEwan develops Briony and Cecilia as foils for one another, which foreshadows their impending rift. The differences between the Tallis sisters—cemented through imagery, metaphors, and similes—fuel the motif of order versus disorder. Whereas Briony loves tidiness and regularity, Cecilia feels at home in the very opposite. McEwan connects Cecilia's cluttered room to her distaste for familiarity and her desire to break free from her family.
McEwan uses descriptions of the sisters' bedrooms to emphasize their differences. In the first chapter, the reader gets an impression of Briony through her personal space.
Whereas her big sister’s room was a stew of unclosed books, unfolded clothes, unmade bed, unemptied ashtrays, Briony’s was a shrine to her controlling demon: the model farm spread across a deep window ledge consisted of the usual animals, but all facing one way—toward their owner—as if about to break into song, and even the farmyard hens were neatly corralled.
Although Cecilia is not yet introduced by name, this instance of juxtaposition is the first time she's mentioned in the narrative. The imagery of the big sister's cluttered room, a metaphorical stew, is the complete opposite of Briony's metaphorical shrine. Nevertheless, the reader's first impression of the younger sister is not necessarily more favorable than that of the older, as the diction of "possessed" and "demon" make Briony's preference for order seem overzealous. In fact, the imagery of the miniature animals all lined up perfectly can almost feel disturbing. This uneasy impression extends to the description of Briony's dollhouse, where the dolls are positioned as though they were "a citizen’s army awaiting orders." The narrator goes on to connect Briony's love of order to a lack of secrets: "Her wish for a harmonious, organized world denied her the reckless possibilities of wrongdoing." This will become important, as Cecilia's more chaotic tendencies come to be associated with her rich and complicated inner life. It also foreshadows Briony's looming wrongdoing.
In the second chapter, the narrator focuses on Cecilia, confirming the impression the reader has already formed: "Cecilia knew she could not go on wasting her days in the stews of her untidied room, lying on her bed in a haze of smoke." It appears that Cecilia is fully aware of her mess but doesn't mind it. What bothers her is familiarity; she longs to escape the comfortable order of her home life. As she stands by the fountain, for example, the view down the drive gives "an impression of timeless, unchanging calm which made her more certain than ever that she must soon be moving on." And later in the first part, when she helps the twins get dressed for dinner, a vague resolution enters her thoughts: "she had to get away." In this chapter, order and familiarity take on a negative connotation. Cecilia feels confined and stifled by the harmony of the Tallis household.
Order and disorder continue to play a central role in the novel, especially in Robbie's experiences from the war and Briony's experiences at the hospital. Moreover, McEwan suggests that Briony's penchant for order is her reason for writing, as storytelling is a way for her to create order in a tumultuous world. In fact, the novel ends with an elderly Briony instilling order in her life, as she prepares for sickness, memory loss, death and the end of her story.