During a conversation with Deadline about tonight's episode of Arrow, showrunner Marc Guggenheim was inevitably drawn to talk of DC's Legends of Tomorrow.

Inevitably, because tonight's episode featured a guest appearance by John Constantine, and with this year's The Flash/Arrow crossover set to introduce Vandal Savage and launch Legends, it seems like a pretty natural train of thought.

What's the biggest difference between this year's two-night event and last year's? Well, for starters, it's an actual crossover event.

"There’s a lot of things that are really cool about this year’s crossover, one of which is the fact that there really is a true 2-part story unlike last year where it was two interconnected episodes where each had their own beginning, middle, and end," Guggenheim explained. "This is a big two-part event, one single bad guy, and it will very clearly launch Legends Of Tomorrow. But at the same time what I think is a lot of fun about it is that it doesn’t feel like a backdoor pilot for Legends Of Tomorrow. It feels like an episode of Flash and then an episode of Arrow. It truly is a 2-hour Flash/Arrow event that just so happens to introduce us to some new characters who will be a big part of Legends Of Tomorrow."

The episodes will air the week before the midseason finale episodes, both of which are Christmas-themed, so probably the last week in November or the first week in December.

Arrow airs Wednesday nights at 8 p.m. ET/PT on The CW. The Flash airs Tuesday nights at the same time.