In his CBR Speakeasy interview, "Arrow" co-showrunner and executive producer Marc Guggenheim discussed John Constantine's guest appearance on the series, revealing that the crossover was planned before "Constantine" was cancelled by NBC, and always as a one-off.
"Back when 'Constantine' was on, we were talking about a crossover. Then 'Constantine' wasn't on, so we stopped talking about a crossover," Guggenheim told CBR. "It took a while. I think it was actually something Matt Ryan and Stephen [Amell] started talking [about]. It was always in the cards."
Guggenheim clarified that Constantine's appearance was always meant to be a one-time thing: "When we were given the character, we were given the character with the understanding and the agreement that this is a one-off."
"Arrow" airs Wednesdays at 8 p.m. ET/PT on The CW.