She is first seen rushing towards the building's elevator with her daughter. She asks Arthur, who is inside the elevator, to hold the door, which he does. While the elevator moves upwards carrying them three, her daughter keeps pestering her with childish behavior. Sophie then looks at Arthur and humorously mimics a gunshot suicide through gestures and grimaces, apparently to express that she cannot put up with such annoying behavior. After the elevator reaches their floor and the two proceed to separate ways towards their respective apartments, Arthur calls Sophie's attention with a "hey!"; when she comes to a halt and turns back to listen to what he has to say, he imitates a gunshot suicide in the same way she did in the elevator. Sophie reacts with confusion and an insincere smile and then continues her way. Arthur becomes obsessed with her and, the following day, decides to follow her to the bank where she works.
It is implied that, from this point on, Arthur starts hallucinating about having a relationship with Sophie. She rings the bell of Arthur's apartment to ask if he had been following her. Arthur is astonished to see her there and confirms that he had. She says that it seemed he was intending to rob the bank. Arthur responds by saying that he has a gun and could "come in by tonight". Sophie interprets this as a joke and bursts into laughter, revealing that she is actually sympathetic towards Arthur instead of scared or angry. He says he is a comedian and invites her to his stand-up comedy show. She asks him to let her know about his next performance.
Arthur sees Sophie at the show, but he bombs due to having no medication to control his own condition that causes him to laugh at inappropriate times.
At one point, Arthur rings the bell of Sophie's apartment and, after she opens the door, he kisses her promptly. Sophie appears to enjoy that surprise.
After Arthur murders three young, rich men who bullied him in the subway and flees the scene, his crime appears in the headlines as having been perpetrated by an unknown man with a clown mask or makeup. As Arthur and Sophie observe newspapers with these headlines displayed on the street, Sophie, not knowing that Arthur was the one responsible for it, comments that she believes the murderer clown was "a hero" and that the city's self-conceited elite deserves it.
When Arthur's mother, Penny, is hospitalized in a critical condition, Sophie is seen accompanying Arthur to the visits and comforting him.
After several incidents continue to erode Arthur's sanity, he lets himself into Sophie's apartment. Frightened, Sophie asks if his name is Arthur and politely asks him to leave. It is suggested that Arthur's past interactions with Sophie were all delusions that he was suffering.