in the Washington City Paper:
WCP: Along with location, the film’s pop-heavy, throwback soundtrack is also one of the most striking elements. Can you talk about the motivation behind the music?
NB: I wanted the music to be big and romantic and celebratory and hopeful. I wanted it to feel good. That was the case with all the pop songs I used, they were all songs that I would always play and immediately want to play again as soon as they’re over. It’s an almost indicative quality to a great pop song. And I think Greta and I, in some ways, always thought of the movie as a pop song. A great pop song is in some ways like a great movie: Something that you could watch and as soon as it was over just play again. So I picked songs that that really gave me that feeling.
more (he also explains the black-and-white thing, although I still wish it had been in color)