When actor Kyle Soller saw the film Rogue One: A Star Wars Story, he was impressed with the grounded story and liked how it combined many different genres. When Soller talked about the role of Syril Karn with writer Tony Gilroy, he wanted to know where Karn ended up. The actor thought that the character was very complicated and could go either way. Gilroy said he did not know, but Soller believed he did know and trusted him with leading Karn to where he felt he was going. Soller also thought Karn knew what he wanted but lacked the necessary emotional acuity and development.
Knowing that Gilroy worked on Rogue One and wrote Andor, Soller read the script and saw how Gilroy fleshed out the show's story. He liked how Karn was three-dimensional and in a gray area. Soller was surprised by the script's grown-up and gritty tone, and he looked back at the title to make sure the show was Star Wars. He was amazed that Karn hit rock bottom early in the show, and he wondered where the series would go from there. He thought that the scene of Karn coming home to his mother, Eedy Karn, was "so un-Star Wars-y." Soller ended up playing the role after being captivated by the moral ambiguity Gilroy included in the Andor television series.
Kyle Soller as Syril Karn in his character poster
Soller thought it was wonderful that Karn had a trajectory composed of two character arcs. He also thought that Karn believes he belongs in the higher structures of the Galactic Empire at the ISB. Soller saw Karn as methodical and meticulous, and he used Karn's OCD to show that he would try to control and structure his environment. He thought that Karn's OCD was a perfect match because detectives with OCD are more effective at their job. Soller had all of his costumes tailored for him. Soller always explored that Karn did not know where the lines are, and he had a deep emotional well of frustration and anxiety hiding below his facade.
Scenes on Ferrix were filmed in an intricate set in Little Marlow, Buckinghamshire. According to Tony Gilroy, everything on Ferrix was filmed first. During filming, Soller forgot that he was in Star Wars due to viewing the show as a sociopolitical drama. When he was walking through the Ferrix set, the crowd parted to reveal a line of stormtroopers, causing Soller to have what he called a "childhood freakout." Soller said working with the droid B2EMO was a big moment for him. Soller believed that the only time Karn properly took in other people's thoughts or lives was when he gave a speech to members of Pre-Mor Enforcement in the episode "Reckoning."
Soller fabricated a story about Karn's father, and he started collaborating with Kathryn Hunter, Eedy Karn's actress, about the story. They both figured out that he left very early on in "a real acrimonious, horrible way." They thought that being wronged was the catalyst for how Eedy raised her son. They believed that Eedy's anger, grief, and disappointment in the father leaving filtered into Karn. For the episode "The Axe Forgets," the props department asked Soller if there was anything he did not eat, and he replied that there was not. They then told him the question concerned the cereal that they showed him, which he described as "Star Wars Cocoa Puffs." Soller said that the cereal was the best thing ever and "tastes like dreams and unicorns." He viewed the cereal as a symbol of Karn's childhood and an indication of how much Eedy does not know Karn. During the conversation between Karn and his mother, Soller attempted to think of different ways to annoy her.
When Soller first met Dedra Meero's actress, Denise Gough, and filmed their scenes together, both actors tried to match each others' intensity and ambition. Soller and Gough were brought together by their shared OCD. Soller believed that Karn kept Cassian Andor's image because it powered his will to survive, which was ignited even more when he first met Meero in the episode "Nobody's Listening!." During that scene, Soller thought that a fire rekindled in Karn due to the characters sharing ideals and instincts about Andor and the expanding rebellion. Soller also thought the scene was the first time Karn felt seen and recognized. The actor did not understand how deep the Star Wars community is until press started months before the show's release. In September, 2022, he attended the Andor launch event in Hollywood, California.