That started with trying to work out what music could be used to describe the character of Reynolds [the designer]. I said to Paul, we should think about what music he would have listened to. I sold Paul the idea of Reynolds being into Glenn Gould, that Reynolds listened intensely to all those recordings. It was a good excuse to work on a slightly neurotic Bach-piano-style piano music, which seemed to fit him. And then Paul asked for it to be bigger and bigger and bigger. So eventually we did it with a full orchestra.

In the piece from the film “Sandalwood I,” I heard Steve Reich stuff, and also Debussy.

Things leak into my head, I suppose. But I’m constantly stumbling with this stuff. I will spend weeks and weeks on the piano, and things start to be satisfying, and then you realize later where they’ve been stolen from.

The harmonies in “The Tailor of Fitzrovia” suggested to me a composer you haven’t mentioned: Britten. Are you a Britten fan at all?

Lots of it was coming from Paul as well as from Daniel [Day-Lewis, who plays Reynolds]. We talked about music in Britain in the ’50s, and did lots of research, and among all this slightly twee, pastoral, folksy stuff, Britten really stood out for having the darkness to it.

So Daniel Day-Lewis gave input for the music?

He talked about Thomas Tallis a lot. How can music sound English and still be Romantic? One piece, “Alma,” for string quartet and piano, is the most English of everything. I’m not sure why. It just has that kind of harmony to it. I feel like it’s the closest we got. I’m lucky Paul used it in a very tender, romantic moment in the film, at the New Year’s Eve party.

The film also includes excerpts from pieces by Debussy and Schubert and more. Were these your choices?