Since the release of Brooklyn (2015), the consensus has been glowing in terms of the film's loyalty to the Colm Tóibín novel. In an interview with The Washington Post, the book's author himself said of the film, “There’s….an authenticity about it.” But while the movie is a faithful visualization of the book’s story, its success in translating the narrative into a new medium is also due, in part, to its deviation from the novel. During a November 2015 Q&A hosted by Irish Screen America at the Crosby Hotel in New York, Director John Crowley observed that there are two key differences between the novel and the film: the agency of Eilis' character and the ending.

Crowley noted that the agency "has been dialed up" in the film adaptation of Brooklyn, which transforms an internal novel into a dynamic, externalized movie. Where the novel's version of heroine Eilis Lacey is passive and less directly in control of her fate, Saoirse Ronan’s take on the character depicts a young woman who gradually takes ownership of her life to shape the outcome of the film.

Crowley described the film as the "story of a young women coming into her agency," a theme that is far more pronounced in the film. Throughout the movie, Eilis transitions from a scared, uncertain immigrant to a capable, professional New Yorker. This journey is emphasized by body language that signals growth in confidence, an acryllic color palette and increasingly vibrant costumes that bloom along with the character as she begins to gain mastery over the course of her life.

A key scene that emphasizes this increase in agency is the conversation with Eilis' former employer, Ms. Kelly. As Crowley said, "In the novel, the scene with Ms. Kelly is less confrontational -- that scene is more about shaming. She gets scared... she sort of runs away and has to run back to America because her secrets are about to come out. [In the film], when you're dramatizing something which is genuinely about a clash or conflict of values, it felt like something needed to be fought in that scene." In the film, when confronted by Ms. Kelly, Eilis rises triumphantly and proclaims, "My name is Eilis Fiorello," making the voluntary decision to return to America and her new husband.

The fim's ending, too, gives Eilis' story a more conclusive -- and arguably happy -- resoultion. Toibin ends the novel with Eilis on the train (a scene that is depicted near the end of the film), suggesting that she plans to leave Ireland but surrendering the details of Eilis' fate up to the reader's imagination. The film continues past this scene to show Eilis' happy reunion with Tony. Crowley said at the Irish Screen America event, "If there's a point to watching it for an hour and a half, it felt that we needed to deliver her. [It's] not a happy ending because you couldn't watch the scene with her mum or the scene with the young girl on the deck of the ship and [the plot with] Jim and say it's a happy ending -- because there's such a lot of cost. But it felt, dramatically, it did need to push back action-wise and land back in America. Colm Tóibín... paid [screenwriter] Nick [Hornby] the highest compliment. He said that, as a prose writer, he felt he could never deliver her back to America because it would betray the reader. So much of the reading is about leaving it to the reader's imagination that you hand the rest of the story over for you to figure out what happened next. But he felt that if he were writing a screenplay, he likes to think he would have smiled at the golden moment where he realized he could send her back to America, and back to Tony. And you deliver it back in a positive way. But it's got a lot of emotional cost."

The differences between Brooklyn's novelistic and cinematic forms correspond with fundamental differences in how audiences experience cinema and literature. In the book, the reader is privy to Eilis' thoughts and ideas, understanding her in terms of how she perceives the world around her. In the film, the viewer's understanding of Eilis is shaped by subtle visual cues that show Eilis demonstrating a boldness that battles with her vulnerability. While these cues are the collective result of screenwriting, directing, and set design efforts, they stem primarily from Saoirse Ronan’s choices as an actor. Ronan’s Eilis walks a fine line between being capable and shaken by her surroundings, which highlights Eilis' internal struggle for control over her life.

Many smaller details also changed during the adaptation process, from the elimination of Eilis' older brothers to altering a central scene so that, in the film, the landlady, Mrs. Kehoe, does not hear Eilis sneak Tony into her room at night. Most importantly, though, Ronan's subtle yet powerful performance creates a big-screen Eilis Lacey that is inspired by the novel but a distinct creation of her own -- a young woman who achieves the bravery and confidence to decide her own fate.