In the Opening scene of Episode 1, a little girl resembling Haruhi from the Haruhi Suzumiya Franchise can be seen in the crowd. Both shows are from Kyoto Animation.
When Evan Call was approached to score for the show, Call read the novel at the request of Taichi Ishidate to draw inspiration from, given it was still very early in production and no official materials had been made yet, save for materials created for animated previews for the novel. Call's sample piece that he submitted for consideration became the main theme of Violet.
Leiden is a city in the Netherlands.