Director Park Chan-wook has mentioned that initially he didn't like the idea of using many text messages in the film. He even said that he considered making it a period piece, in order for the characters to write each other beautifully hand written letters instead of text messages on their phones. When he finally decided on a contemporary setting, when possible, he had the characters use a smart watch, voice recordings and translation apps instead of typing.
Park Chan-wook's inspiration for the film was the Swedish crime novel series "The Story of a Crime" by authors Maj Sjöwall and Per Wahlöö. The series follows the aging police detective Martin Beck and are known for their social critique. Park's pitch was "what if Martin Beck fell in love with a suspect?"
The complete Martin Beck book series, written by author couple Maj Sjöwall and Per Wahlöö, is visible on a table in a scene as a nod to the fact that Director Park Chan-wook have stated that he got inspiration for the film from those books.
Tang Wei did not speak Korean prior to being cast in this film and was required to learn the language for her role.
In a 2023 interview with Deadline, Park Chan-wook spoke about how the film took indirect inspiration from Vertigo: Aus dem Reich der Toten (1958): "I've previously said that I wasn't conscious of the movie Vertigo when I was making Decision to Leave, but I did also mention that it did have some subconscious influences on the making of this film. And when I say influences, I don't necessarily mean the similarities between the plot or the characteristics of the plot. What I meant is... this daydream like quality of the film. That was the influence, subconscious influence of Vertigo on the film. I think this is especially the case in part two of the film, which takes place in Ipoh because of the mist in the city. And from beginning to the end, what I really intended was to make this a dreamlike world, where you feel lost and you don't know which way to go and everything is unclear. That kind of psychological state, where you're still searching for something, you're still searching for clarity. That was the kind of dreamlike state that I intended to create here."