Kira was a last-minute addition to the cast. The original plan was to include the Star Trek: The Next Generation (1987) character of Ro Laren, but Michelle Forbes didn't want to do a series at the time.
When Colm Meaney was fitted for his Deep Space Nine uniform, he made two requests of the costume designers. He explained that unlike the officers, the non-commissioned Chief O'Brien was a working man. So he needed to be able to roll up his sleeves, and he needed pockets for his tools. The costume department altered his uniform accordingly.
Although we only rarely see it, there is an ATM in Quark's bar. It dispenses the various types of currency used by major races visiting the station: Federation credits, Bajoran litas, Cardassian leks, and Ferengi latinum.
Colm Meaney was initially reluctant about signing onto the series. Meaney was comfortable playing O'Brien on an episode by episode basis for Star Trek: The Next Generation (1987), and at the time, was unsure if he wanted to play a full time television role.
Constable Odo was originally envisioned as a young Clint Eastwood type. When Rene Auberjonois was called in for his audition, the casting director told him that none of the previous actors had been "grouchy enough". So Auberjonois improvised his lines using his most gravelly voice, and secured the role. Odo's scoff eventually became such a character trademark that the screenwriters would often script it into his lines (as "harrumph!"), much to Auberjonois' annoyance.