Koreeda said that all the characters in the film, to some degree, are imprisoned by invisible walls. "The mother is restricted by social norms, and the teacher is restricted by his manhood, as well as the school system. And the children are living in this world that the adults have created and they are affected by it, in terms of the violence that is perpetrated upon them. Yet they are able to escape it. It's as if the adults have been left behind, and the children have gone ahead and come to their own self-realization. I wanted it to be a positive realization."
According to Koreeda, whenever he worked with children in his films, he would normally select the child actors who would be good for the roles and, then, edit the script to match the characters' personalities with those of the children. The children even get to add their own words to the script so that their dialogues would sound more natural. However, for this film, Koreeda thought it would be dangerous to work that way as Monster involves a delicate subject matter. When reading the script with the producers, they realized that the actors who were going to play Minato and Yori would have to play them as distinct characters from themselves, so Koreeda changed his approach and didn't choose actors for their personalities or edit the script to fit their personalities.
As for the interaction and friendship between Minato and Yori, Hirokazu Koreeda said he wanted to show a contrast between the boys when they're with adults and when they're together. When they're around their parents or even in school, Kore-eda wanted to show an oppressive world where they can't be themselves. "Their freedom comes when the boys are together in nature and away from home or school. The natural environment, when they're playing outside, walking and even playing, is lit by sunshine and vibrancy - that's where they can be themselves and be true to who they are."
Hinata Hiiragi, who plays Yori, is scared of bugs. Consequently, he had to stay on high alert and even ruined takes while shooting certain scenes in the forest of the abandoned train.
Ryuichi Sakamoto's collaboration with Hirokazu Koreeda for the music in Monster (2023) marked the first time in years - and also the last time - that the legendary composer behind O Último Imperador (1987) and O Regresso (2015) had worked on a Japanese title. Sakamoto died on 28 March 2023 after a long battle with cancer. "It's a great loss to have such a major figure in the music industry gone now, but I was very honored to have been able to work with him on his last film," Koreeda said.