For me every element of River of Fundament is music. Part of the idea of not showing it in cinemas is to encourage it to be experienced more as an opera, to understand the music as something more primary than a film score. All the elements of the film are inseparable, and every sound is part of the musical landscape - any environmental or foley sound can be heard as music. But there’s also something “musical” in the visual sense of the film, so that any movement in the film has a kind of musical rhythm and is as important in that way as the musical notes that you hear, that the characters sing and play. It’s a luxury, and it’s a wonderful thing about the piece, that there’s a degree of abstraction in the narrative that means that music itself is allowed to be one of the main characters.

That's particularly the case in the forging scene in Detroit, where the whole set becomes a musical instrument, and metal “strings” are flung from the top of the smelting furnaces to the forge-pit below, and are played by the furnace workers. But there are also places where a more straightforward dramatic narrative takes over, and that balance between abstraction and story creates a huge available space for different kinds of relationship between the music and the film. You couldn’t have that in an action drama!

Matthew [Barney] and I work absolutely collaboratively. It’s difficult for people to understand - in the art world the audience wants to relate to a single author. But we work in parallel, so that we don’t even know what the other one is doing. We both tend to be inclusive rather than reductive; that means that if I’m working out my musical ambitions and he’s working on his side, when we put them together, it can be bigger than either of us thought it would be.

The origins of this project came when we were working on the Cremaster Cycle and we started talking about the idea of our next project being live performances rather than a film. I find film a difficult medium: it’s my worst hell to think that you try things out with the performers, encouraging them to be as experimental as possible and then you end up with a situation where the surprises can’t happen again because they’re fixed for all time in a film!

But in The River of Fundament I was involved in the picture editing, and we arrived a way of making things that means there’s enough complexity in the finished film that you can never listen to it the same way twice. Sometimes I would be the first person to cut a scene, to show how the music would fit together. And fitting the music together is also determining how the picture should be. It was a long, but fun, process.

The idea for basing the piece on Norman Mailer’s Ancient Evenings was Matthew’s. Mailer himself presented it to Matthew as something he could use as the basis of a project. I remember being in my studio in Berlin and looking at the first 100 pages of his novel, and being immediately excited. I thought what a wonderful opera this could be, this situation where someone describes coming to consciousness to find themselves dead and about to be mummified. But I took clues from the story rather than interpret it literally. I like to move behind the sense of a specific narrative, to work in this inbetween world in which there is no essential dramatic logic. That’s where I can work intuitively in my music - but then it’s good to have something to check back into, to relate it to. Matthew stayed more with Mailer’s narrative than I did.

The foundry scene was actually a nice case of the things you can do in the medium of film and what you can’t achieve so easily in the moment of live performance. In the editing, we created what should have happened; a fantasy version of the scene that didn’t happen in the real time of performance. We’ve got the trombone choir on the scaffolding above the furnaces, the saxophone groups and the percussionists and the singers, and those metal viols who could barely squeak out a sound in the weather – it was pouring with rain when we were shooting! Thankfully I had picked up recordings in rehearsal so I was able to compose together a mix of those recordings, and we could create an ideal version of what the scene should look and sound like.

The whole project reflects the most important thing for me, that the music comes from within the film, that it’s not something that was added later. Music is built into the fabric of The River of Fundament, as performance, as idea, as a set of relationships between what you see and what you hear, and as abstraction and narrative.

• River of Fundament is screened at the Coliseum, London on 29 and 30 June.
Jonathan Bepler was talking to Tom Service.

See also

Adrian Searle interviews Matthew Barney: 'My work is not for everyone'