It struck me that it might be interesting for once to do an almost blue-collar warlock. Somebody who was streetwise, working class, and from a different background than the standard run of comic book mystics. Constantine started to grow out of that.
Source, in full
On the creation of the character John Constantine in Swamp Thing, as quoted in "The Unexplored Medium" in Wizard Magazine (November 1993); the character he created later appeared in other works, including Books of Magic by Neil Gaiman, and his own series Hellblazer.