'Arrow' Season 4 Constantine Crossover: Showrunner Wendy Mericle Shares More Details

Fans of the CW's Arrow have had quite a week, given all the new casting additions for season 4. The news that Matt Ryan would appear was perhaps the biggest news and now, Arrow showrunner Wendy Mericle has given even more details about the Constantine crossover.

Ryan will appear in the fifth episode of season 4 called "Haunted." We have already learned that John Constantine's appearance is tied to the resurrection of Sara Lance (Caity Lotz) as White Canary.

Recall the trailer for Legends of Tomorrow in which we saw Sara emerge from the Lazarus Pit, the healing fountain introduced in Season 3 of Arrow that has the power to mend wounds and even bring people back to life. From the trailer, it seemed that this would be how Sara returns as White Canary, but executive producer Marc Guggenheim hinted that there's more to it than that.

"Unlike Thea, who wasn't fully dead, the consequences of using the pit on someone as dead as Sara was creates some unforeseen difficulties," he said, according to TV Insider. "And John Constantine is uniquely suited to help our team deal with those."

Mericle expanded on how Ryan's appearance came to be in an interview with IGN.

"It was born more out of the storyline for Sara Lance," she said. "We needed someone to come in and help bring her back. He felt like the most exciting possibility and we're just thrilled to get him. He brings such a different element to the show."

And if any fans of Constantine are concerned that the character will be changed at all from the series, Mericle assured them not to worry.

"He is coming in fully as who he was on the show," he said. "We're getting wardrobe. We're very excited to have him. He's a tremendous actor. It's so cool to have that cross-pollination. We are very lucky and DC was very generous letting us have him."

Finally, CW president Mark Pedowitz had said that Ryan's appearance would be a "one-off," and IGN asked Mericle if that is truly the case. "Right now, it really was -- it's born out of necessity for this particular story," she replied. "I think Mark was right, it is a one off and this is what we're focusing on right now."

Constantine does seem like a prime candidate to return later in Arrow Season 4 (given the theme of magic and mysticism), or even as a full-fledged character on DC's Legends of Tomorrow. And of course, there's always The Flash.

What do you think of Mericle's remarks? Let us know in the comments section.