This is an interview done by ANN:

ANN toured Wit Studio’s office in Tokyo and spoke with director Masahiko Ohta in person—and producer Naokado Fujiyama and promotional producer Aya Okano via email

Probably the thing that surprised me the most was that Koshitan’s song was improvised by the VA (quote by Fujiyama):

I personally love the [musical] gag ‘Go forth, former delinquent Koshitan!’ that was originally in chapter 17 of the manga and was shown in episode 3 of the anime. In the original, it was a relatively short gag that ended quickly, but in the anime, it was expanded in length. I also think that having Saki Fujita [Koshi’s voice actress] sing this song that she ad libbed on the spot makes the inherent charm all the more apparent.