Throughout the novel, the narrator suggests that Briony occupies an uncomfortable and lonely position between childhood and adolescence. Briony herself, as well as the other characters, are aware of this transition, which comes with big expectations surrounding selfhood, maturity, and insight. In Atonement, growing up is often depicted as a sort of limbo, and becomes a central motif in the characters' understanding of each other's behavior. The many mentions of Briony's age in the early chapters foreshadow the far-reaching consequences of her rushed pursuit of being admitted into what she sees as an "adult cabal."
The arrival of the cousins sparks Briony's reflections on her age and maturity. When she interacts with Lola in the first chapters, Briony takes particular note of her own childishness. Lola dresses "in the guise of the adult she considered herself at heart to be." When they rehearse for the play, she does not reveal "any demonstration of ragged, childish enthusiasm." Although Briony can tell that Lola is consciously playing the role of the older cousin, it still impacts how she sees herself and her own maturity: "She thought how she should take more care of her appearance, like Lola. It was childish not to." Lola's behavior induces Briony to reflect on her age, knowledge, and access, which further shapes her perspective on the interaction that unfolds between Cecilia and Robbie at the fountain: "This was not a fairy tale, this was the real, the adult world in which frogs did not address princesses, and the only messages were the ones that people sent."
The domino effect of Lola's teenage posturing has further consequences. Just before Robbie gives Briony the fateful letter, she symbolically purifies herself of childhood. As she slashes at nettles, Briony imagines that she's severing "the sickly dependency of infancy and early childhood, and the schoolgirl eager to show off and be praised, and the eleven-year-old's silly pride in her first stories and her reliance on her mother's good opinion." When Robbie gives her the letter right after this rite, she comes to identify it as an initiation. After reading the letter, she takes "the very complexity of her feelings" as confirmation that she "was entering an arena of adult emotion." She notes that "It was a chilly sensation, growing up," and realizes with some sorrow that she will "never sit on Emily's or Cecilia's lap again, or only as a joke."
Several of Briony's ensuing experiences confirm her conviction that she's grown up. For example, she views Robbie's hatred as "another entry": "to be the object of adult hatred was an initiation into a solemn new world." And in the legal process that unfolds after Briony frames Robbie, the experience of being "listened to, deferred to and gently prompted seemed at one with her new maturity." Ultimately, however, Briony comes to realize that she didn't grow up over the course of this single day. Rather, it's over the course of her many years of guilt and atonement that she leaves childhood behind. When Robbie asks her in the third part what made her change her mind, she tells him "[g]rowing up."
Throughout the first part, various characters reflect on Briony's awkward pubescent position. Robbie thinks to himself that she inhabits "an ill-defined transitional space between the nursery and adult worlds." Emily identifies Briony's "slow retraction, the retreat into autonomy" as a signal of "the approaching end of Briony's childhood." In the third part, the rejection letter echos this view of Briony: "We catch this young girl at the dawn of her selfhood."
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.
Because the narrator moves between the perspectives of a range of characters, the novel contains many instances of dramatic irony. Moreover, because the narrator is narrating with the benefit of hindsight, the first part also contains a large amount of foreshadowing. The characters' discordant views of situations makes the reader expect conflict.
In the ninth chapter, Jackson tells Cecilia that Briony is gone. At first, Cecilia's concern for Briony snowballs into concern for everyone else in the house. She dissipates this concern by reminding herself that the outcomes one imagines in moments of anxiety are never what actually ends up happening:
This time she paused to peer out of the window at the dusk and wonder where her sister was. Drowned in the lake, ravished by gypsies, struck by a passing motorcar, she thought ritually, a sound principle being that nothing was ever as one imagined it, and this was an efficient means of excluding the worst.
As an older sibling who has often had to act as a parent, Cecilia knows that no matter how much one plans and worries, things never turn out quite as one expects. Thinking the worst is a strategic ritual through which Cecilia ensures that the worst won't happen, and the fact that the narrative calls attention to this mindset foreshadows that things will go awry later in the novel.
On the one hand, the passage emphasizes Cecilia's clever, thoughtful wisdom. On the other hand, it strengthens the underlying dramatic irony, because the reader knows why Briony is gone. She hasn't drowned, been kidnapped, or been hit by a car. Instead, she has been given—and very likely has read—a letter that directly concerns Cecilia. This doesn't immediately seem as bad as any of the worst-case scenarios Cecilia comes up with, but the reader still knows that it will introduce a great amount of tension into the narrative.