“I wanted to explore human emotions,” Japanese script writer Kosuke Mukai says of his new film Gukoroku: Traces of Sin, the opening film of the Japan Foundation’s 2018 Touring Film Programme.  The story, which is based on the book of the same name by Tokuro Nukui, explores dark characters, the world of the elite, and those desperately seeking to enter it, all wrapped up into a whodunit crime thriller. Directed by Kei Ishikawa and starring Satoshi Tsumabuki and Hikari Mitsushima, it is Mukai’s script that makes the story thrive, and supports Ishikawa’s impressive yet disturbing visuals.

We are both wrapped up in our winter coats, anticipating the moment we have to move our interview from a meeting room in Holborn to a taxi heading to Paddington station. Despite this he listens intently as we ask him questions and replies quietly as he looks from us to the translator, and even as we switch locations he’s more than happy to talk about the project. It makes sense, since the film is a clever exploration of human nature, and presents Mukai’s skills as a writer which he’s previously shown in films like Linda, Linda, Linda and Satoshi: A Move for Tomorrow.

Gukoroku is based on a novel, and director Kei Ishikawa has said it was quite a challenging story to adapt because of the way it’s written. How did you approach the project as a writer?

It’s a mystery, a crime novel, but at the same time it is about human relationships, and particularly these characters that live in a small world. They mess around, speak behind people’s backs, and they’re jealous of each other. They have quite complex emotions throughout the story so what I wanted to do was to describe a society that focuses on class differences, and those born into such a world. Of course, finding out who killed the family is an important element of the story, but it wasn’t just about that.

Since there is this class difference featured throughout the film, what drew you to these dark characters?

I was in Beijing and I saw the class difference in China, if you’re born in an affluent family you maintain that, but if you go to remote villages those that are born in farming families are stuck in that life and can often die in poverty. I didn’t think a class divide existed in Japan, but when I came back I realised that the same things exists there. If you’re in an educated family you will go to top class universities and have good jobs, but I you are born in a lower class or from a poor family then you may end up in the same way. I wanted to show that this exists even in Japan.

In the story the character’s reveal their true nature slowly through the narrative, was this hard to do in the script?

Well the book itself is a compilation of monologues, and people who knew the murdered family confess what they know. The book was very well written and the author gradually reveals these dark secrets that people didn’t know before, so what I had to make sure I did was reach particular points in the story based on the novel and that wasn’t particularly difficult for me.

Was there anything that was particularly challenging for you?

Well the biggest challenge was where to focus the story, because the book features many more confessions than are shown in the film which means that there aren’t any main roles there. My challenge was to find where I should focus, and how I could create a story that looks at a few specific characters.

How involved were you with the filming process?

Of course writing the script was my main role, but in addition to that I was slightly involved in casting and looking at the rushes as part of the post-production process. I was there to see it and make my comments, the things I did [apart from the script] may not have been hugely important but I was there for most of the process. But, of course, the final say goes to the director and producer.

Did you collaborate with Kei Ishikawa for the script?

We discussed things beforehand, and what interested me the most was how Ishikawa said that the husband who was murdered had a huge ambition to get up the social ladder. For Ishikawa he felt that this could be related to The Great Gatsby, and he wanted to recreate this story for a Japanese audience through this film.

Ishikawa trained in Poland to be a filmmaker, was his approach to filmmaking different to other directors you’ve worked with?

It’s very difficult to say, but it’s probably down to Ishikawa’s character. He graduated from a science university, so he’s really scientific and he likes statistics so there’s a lot of logic to how he approaches filmmaking. There are directors like that in Japan, but he probably has that more than others.

You’re no stranger to adaptations as a scriptwriter, how do you choose your projects? And do you prefer working on your own stories?

When I was in my twenties and thirties my focus was to explore what I could do so I didn’t have a criteria for what I wanted to do, I would work on whatever came my way. But one thing I stopped doing is writing manga adaptations, because I did it a few times but the adaptations were not well received so I think that style of writing is not for me. Trying to imagine how a manga character could become a real person is very difficult.

How do you feel about the fact that the film industry in Japan is very focused on manga adaptations?

Personally, I’m not really happy about that. It definitely takes more time but I prefer writing something original, and I prefer creating a character that I think is important to present in a film and that reflects society.

As a writer, what kinds of characters are you most interested in creating?

I’m a friend of the director Nobuhiro Yamashita, and we have both turned 40 and I have been discussing with him about exploring the relationship between husband and wife, and themes like love and family. The film called Husbands by … and that film is close to our heart, and is very inspiring, so we’d like to make something similar to that.

In comparison to Gukoroku, your recent film Satoshi: A Move for Tomorrow is quite different. What interested you in this story?

Originally the director Yoshitaka Mori approached me because he had seen my work before and wanted to collaborate with me, unlike other films that I was involved in this one wasn’t appealing to fundraising because it is about Shogi. So, it took eight years to complete because of these problems, so during that time I kept going back to the script and changing things.

Like Gukoroku it was also based on a book, though it was a non-fiction so was it difficult to adapt?

The challenge for that film was how I could take the story of the life of the protagonist Satoshi from when he was a child to his death at age 29 and fit it into a two-hour film; that was the difficult part. Instead of telling the story chronologically I decided to cut short the early days in his life and focus on the time before he died.

In the film the focus is mainly on the Shogi matches between Satoshi Murayama and his rival Yoshihara Habu, how did you approach these scenes?

There is a lot of detail in the matches, but I’m not a professional so I got an advisor from the shogi world to help me and check that what I wrote was correct. His rival Yoshihara Habu is still a professional shogi player, so although they didn’t have that many matches each one of them has become legendary. There are a lot of records of what happened in the matches so I referred to those archives to help me.

In the film there’s a lot of emphasis on the sound design, especially for the shogi matches, were you involved with this?

There are three matches in the film, and I worked with the director on them all. We decided that each match would have a different focus, so the first is about the sound, the second focuses on picture, and the second is filmed in a style of a documentary so that’s how we created those differences. The director was really fussy about the sounds that were used during the matches.

Gukoroku: Traces of Sin is being screened across the UK as part of the Japan Foundation’s Touring Film Programme. For more information on cities and dates, visit the website

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