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• Elisabeth Moss as Kirby Mazrachi / Sharon Leads, Chicago Sun-Times archivist who was attacked years ago and left for dead^[3][4]
• Wagner Moura as Dan Velazquez, Sun-Times reporter who helps Kirby track down her assailant^[3]
• Phillipa Soo as Dr. Jin-Sook Gwansun (Jinny), researcher at the Adler Planetarium and on Harper's hit list^[3][4]
• Chris Chalk as Marcus, Kirby's husband in one of her shifting realities^[5]
• Amy Brenneman as Rachel, Kirby's single mother^[3]
• Jamie Bell as Harper Curtis, serial killer, and Kirby's attacker^[3]

Recurring

• Christopher Denham as Leo Jenkins, Harper's friend from France during World War I^[6]
• Madeline Brewer as Klara Meiser, Harper's girl friend from after World War I^[6]
• Karen Rodriguez as Julia Madrigal, one of Harper's murder victims^[5]
• Erika Alexander as Abby

Differences from the novel

In Beukes' novel, the story is told from the point of view of each of Harper's victims. Show creator, Luisa made Kirby the principal protagonist and told the story through her eyes. Luisa felt that focusing on a single character's point of view made the show more compelling. She said, "With one character you see all the puzzle pieces ... You're sort of going through this maze with Kirby and discovering the mystery along with her."^[16]

While Beukes downplayed the character of Harper in her book, Luisa fleshed him out a little to make him more intriguing. She was careful to keep him out of the limelight, but said "[h]is humanness made him almost more unsettling."^[16] The show's interpretation of how time works is slightly different from that in the book. Luisa said that she imagined time as being a string that Harper moved up and down on. The closer he is to the top, the more Kirby is affected by his actions. Luisa explained that survivors remain connected to this string, and Shining Girls "is about cutting that string."^[16]

Luisa stressed that she never changed the book's message. "I think Lauren [Beukes] has a very specific worldview on grief and trauma that she presents and carrying that forward was really important."^[16]

Executive producer and lead actor, Moss remarked that unlike the book, in Shining Girls Kirby and Dan do not become romantically involved. Moss said that an eight-episode TV show is not enough time to adequately develop a love subplot. She added, "If you’re going to do it, we really wanted to do it well", and so instead they chose "to hint at it, and the possibility of it" rather than allow it to evolve.^[17]

Other differences between the show and the book have also been reported. In the show, Kirby is ten years older than in the book, and her position at the Chicago Sun-Times is an archivist rather than an intern. In the book the staff at the newspaper know of her attempted murder because they ran the story, whereas in the show Kirby changed her name from Sharon Leads to Kirby Mazrachi in order to keep her assault a secret.^[4]

The shifting realities that Kirby experiences in the show, her desk at work changing position, her cat at home becoming a dog, discovering that she is married, do not occur in the book. Some of the details of Harper's victims are different. Julia Madrigal is killed in 1990, not 1984, and she is connected to Leo, Harper's friend, who is a new character created by the show. The items Harper leaves in his victims are also different. In Kirby's case it is a matchbook, whereas in the book it is a monogrammed lighter.^[4]