Ruth Negga and Dominic Cooper, a couple in real life when the show was being prepared, got into the show together. Negga was preparing her audition for Tulip and asked Cooper to help her prepare. He then read the screenplay and got so interested that started reading the comic books and eventually asked to audition himself for Jesse.
The fourth recent TV show based on a Vertigo property, the other three being Constantine (2014), iZombie (2015), and Lucifer (2016).
The actress playing Tulip, Ruth Negga, is Irish (although born in Ethiopia). The only Irish character, Cassidy the vampire, is being played by an English actor, Joe Gilgun.
When Ennis and Dillon were doing the original comics from Vertigo (mature imprint of DC comics) it is believed that they got away with more violence and gore than was allowed back in the day because of the initial black and white pages they handed to their editor. All the gore and such were added later on in the process of making the comics.