The composer Hans Zimmer was at work on his score for Man of Steel when Nolan approached him [for Interstellar]. “Chris said to me, in his casual way. ‘So, Hans, if I wrote one page of something, didn’t tell you what it was about, just give you one page, would you give me one day of work?’” Zimmer recalled. “‘Whatever you came up with on that one day would be fine.’ I said, ‘Of course, I’d love to.’ One day, an envelope arrived, almost handed to me by Chris. It was on quite thick paper, typewritten, which told me there was no carbon copy. This was truly the original.”
On the paper was a short story, no more than a precis, about a father who leaves his child to do an important job. It contained two lines of dialogue – “I’ll come back” “When?” – and quoted something Zimmer had said a year before, during a long conversation with Nolan and his wife at the Wolseley restaurant in London. It was snowing, central London had ground to a halt, and the three of them were more or less stranded. “There was no movie to be made, there was no movie to discuss, we were talking about our children,” said Zimmer, who has a 15-year-old son. “I said, ‘once your children are born, you can never look at yourself through your eyes any more, you always look at yourself through their eyes.”
He worked on the score for a day and then let Emma Thomas know he was done.
“I said, ‘Do you want me to send it over?’ She goes, ‘Oh, he’s curiously antsy, do you mind if he comes down?’ He got into the car and drove to my studio in Santa Monica and sat down on my couch. I made the usual excuses a composer makes when they play something to somebody for the first time. I played to him, not looking at him, I just stared straight ahead at my copy of the screen and then I turned around and he’s sitting there. I can tell he was moved by it. He said, ‘I suppose I’d better make the movie, now.’ I asked him, ‘Well, yes, but what is the movie?’ And he started describing this huge, epic tale of space and science and humanity, on this epic scale. I’m going, ‘Chris, hang on, I’ve just written this highly personal thing, you know?’ He goes, ‘Yes, but I now know where the heart of the movie is’. Everything about this movie was personal.“ (via)