Last year's Invasion! storyline across the DC/CW shows was billed as a four-part event set over the course of Supergirl, Flash, Arrow, and Legends of Tomorrow. It was a great crossover, but it wasn't really a four-part story, considering the event barely impacted the Supergirl episode of the crossover that kicked things off. Turns out there's a reason for that.
Speaking to Entertainment Weekly, Supergirl executive producer Andrew Kreisberg acknowledged the discrepancy, and offered a completely sound reason for Supergirl's part in the crossover essentially being a post-credits scene between Kara, Barry, and Cisco. Turns out, blocking out schedules for actors across four different shows for a crossover is enough of a nightmare — but doing so when one of the shows suddenly becomes part of your channel is even more of one:
One of the big things we learned from two years ago, which made the crossovers this year slightly easier, was building in shut-down days, where shows just went dark. The single hardest factor in doing the crossovers is actors' availability because the shows keep going on. You're basically juggling four shows worth of schedules. The thing that we didn't really plan on — because we weren't aware that we were going to switch Supergirl from CBS to CW and it was really going to be part of it — that was the one show that didn't have built-in shut-down days, so all the other ones did. Melissa [Benoist] ended up being heavily in Flash and then in Arrow, so Supergirl was the one we needed to shut down the most and it was the one that we didn't have any opportunity to.
The good news is that with Supergirl firmly rooted in the CW family now, fans can expect the next inevitable crossover event to be a story fully told over the four shows — and the shows have been picked up for new seasons, so plans are already underway. According to Kreisberg:
Next year, we're hoping to do a true four-way crossover. The great thing about our dear friends at the CW and Mark Pedowitz picking the shows up as early as they did has allowed us to start building the schedules for next season.
So start your countdowns already: an even bigger CW/DC crossover is on the way, and Supergirl will be a fully-fledged part of it.