When “Arrow” debuted in 2012, who knew it would become the launchpad to an enormous shared universe across four shows? The first show began by introducing Barry Allen in its second season, before “The Flash” became it’s own show. Then the two shows pulled off their first crossover in 2014, and last year “Arrow” and “The Flash” crossed over again while simultaneously launching a third show, “Legends of Tomorrow.”

Now, in what has become an annual end-of-year event, the Arrowverse is poised to pull off its biggest crossover yet with “Arrow,” “The Flash,” “Legends of Tomorrow” and the CW’s newest addition, “Supergirl.”

Variety spoke with executive producer Marc Guggenheim about the stress of planning a four-show crossover, celebrating “Arrow’s” 100th episode and more…

Two years ago was the first crossover; last year, the crossover launched a new show; this year, it’s a three-part crossover with four shows. Are you just trying to find new ways to make these crossovers more complicated every year?

It certainly seems to be what we’re doing. Every year we pull it off and it’s a small miracle, and then the next year we try to find a way to increase the degree of difficulty. Truth be told, the whole thing is really an exercise in insanity and we just keep making it harder and harder and harder on ourselves.

How difficult was it to schedule everything this year across the four shows?

It was definitely hard. The thing we learned from last year was ways to make scheduling more efficient. One thing we did this year was we built two shutdown days into “Flash” and “Arrow” and “Legends.” What that did was alleviate some of the pressure. It didn’t solve all of our scheduling problems by any stretch of the imagination, but it did improve the efficiency of things. From what I’ve been told from the actors, the scheduling actually went smoother this year than last year.

Did you get to be on set when everyone was there and suited up?

No! I wasn’t unfortunately. I tend to go up for the prep. It’s in the prep that I feel I can do the most good — or the most damage.

Why choose the Dominators as the villain for the crossover?

For us, it didn’t start out so much as, “Well let’s do the Dominators,” but it started out as us knowing we wanted an external threat — something that wasn’t tied to the mythology, or big bads, of any of the shows. We wanted it to be something that came outside the four shows to threaten the heroes and the world. From there, we landed on aliens invading, and when you’re in the DC Universe and you talk about aliens, our heads immediately went to “Invasion!” which was a story that didn’t introduce the Dominators, but popularized them.

Aliens are a big focus in “Supergirl,” but not so much with the other shows. How do the characters in “Flash,” “Arrow” and “Legends” react?

They all handle it a little differently. For a lot of these characters, they’ve reached the point where they’ve dealt with time travel, parallel universes and super powers, so were aliens that much of a leap? In the case of the crossover, I think Oliver Queen (Stephen Amell) has the most visceral response to aliens. For him, it might be the straw that breaks the camel’s back, or the bridge to far, for him.

It’s also “Arrow’s” 100th episode. Did that add even more pressure to the crossover?

Yeah — to the point where we were wondering if it was even going to be possible. Greg [Berlanti] came up with a really terrific device that could only be done with the crossover, but works perfectly with the 100th episode of “Arrow” that really allowed us to have our cake and eat it too.

What kind of an impact do the new recruits in “Arrow” play in the crossover?

The recruits play a pretty large role in the crossover — probably a little larger role than people are expecting. You would think that with all of these different characters that the recruits would take a back seat, but they’re right there front and center. One of the fun things was seeing their reactions. I think Wild Dog’s (Rick Gonzalez) reaction is priceless.

Nate just got his costume last week in “Legends” and this is his first major team-up. How does he handle it?

In many respects, he’s got a pretty calm demeanor about it. In some respects, when you have all these characters, you kind of have to pick your spots. You don’t want to have this montage of characters standing there with their mouth agape. Thea (Willa Holland) and Diggle’s (David Ramsey) reaction, because they come from the most grounded show of the three, they tend to get most of those moments.

This is the first time the Legends have been back in 2016 since Flashpoint. Do they notice anything funky?

Yeah. Stein (Victor Garber) is going to notice something pretty funky, to use your word, when they get back.

Speaking of Stein, does the issue of the message from future Barry get brought up?

You’re going to hear more of the message, yes.

Would you consider doing another four-show crossover in the future, or do you want to keep them a little smaller going forward?

Truth be told, I think that’s something that is more up to the network and the audience. If there’s an appetite for it from the fans and from the network, I would think that next year we could do a proper four-part crossover.

The CW crossover begins tonight on “The Flash” at 8 p.m.