SPOILER ALERT: This story contains details of tonight’s Supergirl and The Flash crossover episode on CBS.

“I think that one of the best parts about the show is it already seemed like Grant has been on Supergirl for a long time,” executive producer Andrew Kreisberg says of Flash star Grant Gustin’s crossover appearance tonight on the CBS superheroine series. Now, crossovers are nothing new to the small-screen versions of the DC Universe, but Monday they jumped worlds and networks with freshman Supergirl and star Melissa Benoist welcoming The CW’s The Flash, which is in its second season.

Unlike the past multi-episode team-ups between Arrow and The Flash, which feature two Warner Bros Television-produced series, Monday’s one-off on CBS only flows one way — at least for now. Co-written by Kreisberg and Michael Grassi from a story by fellow exec producer Greg Berlanti, tonight’s “Worlds Finest” episode might be the start of something more if the momentum, will and ratings are there, Kreisberg tells me.

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An EP on Arrow, DC’s Legends Of Tomorrow, showrunner on The Flash and co-showrunner of Supergirl with Ali Adler, Kreisberg chatted with me about Monday’s crossover, how they pulled it off, how it could happen again and that CW zinger.

DEADLINE: It seems so logical once it’s happened, but how did the “Worlds Finest” crossover come together, crossing not just worlds but networks?

World's Finest Supergirl Flash crossover 2 Andrew Kreisberg KREISBERG: I think we always loved the idea of doing it, but it just never seemed feasible. Then a couple of things happened. One was that we really made the idea of the multiverse work on Flash this season, and the audience really accepted that there could be multiple Earths that vibrate in different frequencies. You can’t see them, but there’s an infinite number of Earths, which is a bit of a heady concept, but the audience really took to it.

For us, it helped explain the idea that Supergirl and Flash could exist in the same universe. It also explains how on The Flash show why no one has ever brought up the fact that there’s Superman in Metropolis. So the idea that this Supergirl series takes place on an alternate Earth that’s just like ours sort of gave us license to blend the two shows creatively without feeling like we’ve jumped a shark in some way.

Batman V. Superman: Dawn Of JusticeDEADLINE: So, I have to ask, with Batman V Superman having opened big on the big screen and WBTV’s Gotham being over on Fox, is there a Batman in Flash and Arrow’s universe?
KREISBERG: (Laughs) The answer to that is above my pay grade.

DEADLINE: Of course, unlike with Gotham, it helps that CBS owns a significant chunk of the CW, at least in terms of greasing the crossover wheels.
KREISBERG: Yes, but the other thing I think that happened was just the enthusiasm that was there for it from the fans and the press and even the executives. People were really excited by the process. The people we got into this season, and the more the idea of the multiverse took hold in the audiences’ imaginations, and the more we understood how to produce Supergirl week in and week out, it didn’t seem quite so impossible as it did at the beginning of the year.

DEADLINE: So, are we going to see another Supergirl and Flash crossover or even greater Supergirl crossovers with other DC TV properties?
KREISBERG: Well, we would love that. I think it’s really up to the respective networks and what their appetite is for it, and certainly the audience. If there’s a strong audience response to the Supergirl and Flash crossover, that’s certainly the wind in the sails to try and make it happen again.

We try to never say never when we do anything anymore just because there are so many things that can happen – when we started with the beginning of Arrow, we said we’d never do that. Now here we are four years later with four different superhero TV shows that are all now interlinked.

DEADLINE: In your past crossover, it’s always been CW show to CW show with Arrow, The Flash and even newbie Legends Of Tomorrow — all Vancouver productions. With The Flash shooting up in Canada and Supergirl made here in L.A. and both in the heart of their respective seasons, what were some of the logistics of pulling off this cross-network crossover production?
KREISBERG: Honestly, it took a little fore-planning. We designed The Flash episodes that were shooting during this time, and the audience might not realize this, but they were actually a little bit Grant-lite. We tended to keep all of his scenes in one location so we could give him days off on The Flash both to free him up to be on Supergirl, but then also to give him a little bit of a break.

DEADLINE: You brought him down to L.A. in February, right?
KREISBERG: Yes, Grant came down and shot for three days, went back to Flash. Then he came down again for one last day of filming, which was the big, epic fight in Act 4 in downtown Los Angeles.

DEADLINE: Melissa and Grant are both Glee alums, but they haven’t actually worked together, yet they have great natural chemistry.
KREISBERG: Yeah, I mean they know each other socially, which I think really helped. They’re so alike in the best kinds of ways and we’d seen the chemistry at conventions and stuff. And after the first five minutes of shooting out in the desert you could already tell it was like they had been doing this forever.

I think that one of the best parts about the show is it already seemed like Grant has been on Supergirl for a long time.

flash arrow crossoverDEADLINE: All in all, it sounds like it was a fairly streamlined process, no?
KREISBERG: Truth be told, the regular crossovers between Arrow and Flash have proven to be far greater logistical situations because with those shows you have people crossing back and forth between both shows. Here we only had a one-way crossover. It was only Grant having to go to Supergirl. So in some ways, even though it was across networks and it was deep in the middle of the season, it was actually a little bit easier.

DEADLINE: Even though this is a crossover that occurs mainly on the CBS, are we going to see any reference to it on The Flash?
Flash March 29 2016 epKREISBERG: Yes, you’ll actually see the moment when Barry went on this experience and when he came back, for sure, in Tuesday’s episode on the CW — which is exciting. We had to flip because of when the shows are scheduled but otherwise the effect that it has on Flash I think is going to be minimal because that was what we were asked to do. We were just happy to have The Flash be on Supergirl.

DEADLINE: Well, the CW makes an appearance of sarcastic sorts on CBS and Supergirl tonight, with the “You look like the attractive yet non-threatening racially diverse cast of a CW show” line whipped off by Calista Flockhart’s Cat Grant looking at Melissa, Grant, Jeremy Jordan and Mehcad Brooks.
KREISBERG: That was actually one of the first lines I came up for this episode. What’s fun about the crossover is just because of the fact that you’re crossing over between two different TV shows, these episodes tend to feel a bit more heightened and you can tend to get a little bit more meta with them. Because when you put two TV shows together it does become a little bit of a meta-commentary on the differences between the shows.

With the first Flash/Arrow crossover, when Oliver was making fun of Barry for coming up with silly code names for his villains and then Berry replied, “Maybe we can talk about it over coffee with the Huntress and Deathstroke.” Getting to have those kinds of fun lines in these shows is definitely one of the most enjoyable parts of the writing experience. So getting to have a little bit of a meta-comment line like that is definitely part of the fun for me.

DEADLINE: Now that you guys have done a few of these superhero crossovers, is there a format that you use to keep all the pieces in place?
KREISBERG: What tends to happen with the crossovers that works really well is when we keep it to one main story that everyone is involved with as opposed to the typical structure where is there is an A story, a B story, and a C runner. It tends to keep everybody involved because there’s nothing more important going on than the fact that the heroes have teamed up.

What’s great about Supergirl at this point in the series is that the characters have become so well defined that James would be jealous and Winn would think Barry was really cool and found a new best friend and Calista would be slightly suspicious of him and figure out his identity in two seconds. Everybody’s reactions were sort of perfect and on point. It made the episode, I wouldn’t say easy to write, but it was certainly a lot of fun to write.

supergirl-1280jpg-e43dd4_1280wDEADLINE: Speaking of Supergirl and the characters on that show being well defined, what can we expect for the season finale April 18?
KREISBERG: We’ve got some amazing set pieces. We’ve got some amazing fights. We’ve got some incredibly emotional scenes. We’ve been telling one long story this season about how Kara Danvers becomes Supergirl.

She’s had her fits and her starts and she’s had her successes and her failures. In the finale you’re really going to see her truly be the best version of Supergirl that she can be because she saves the day not just in her heroics but also with her heart.

DEADLINE: Greg Berlanti has let it drop that you guys have a possible Supergirl role in mind for Lynda Carter, the original TV Wonder Woman, in the future as the President of the United States. The crossover, the appearance of Lois & Clark’s Dean Cain and the big-screen Supergirl Helen Slater on the show, the building blocks of Legends … it seems like you, Greg and Marc love the legacy of the DC Universe?
KREISBERG: Well, we consider ourselves blessed that we get to be the stewards of this leg of these heroes’ journeys. There was a Flash and a Supergirl before us and a Green Arrow before us and there’ll be a Green Arrow and a Flash and a Supergirl after us. We’re just the lucky guys who get to be in charge for this part of the journey. We’re thrilled to invite actors who portrayed some of these classic parts or been involved to be on our shows because it is a sense of history and a sense of belonging and a sense of connection. We’re fans first and for us we’re fans of all the previous incarnations.