Jack had grey fur and black stripes. He had a spy suit, in a nod to espionage films such as James Bond.
It is assumed he was a serious character.
During early development of the film, Jack was supposed to leave the city of Zootopia under the command of his boss where he would explore a world of dangerous animals. This led to Byron Howard's idea of films about rabbit spies. During development pitch, the idea for a modernized all-animal city in the first act, as the city was to be the film's setting. For the final version of the film, Jack was eventually replaced by another rabbit protagonist named "Judy Hopps".