Last night we got the two-hour conclusion to Crisis on Earth-X, which is likely the biggest thing they've done in the Arrowverse… until next year's crossover. The video below is a second behind-the-scenes featurette that includes the cast talking about the filming of the project. They were doing 15 to 16-hour days and at times had to be on set of all four shows on the same day. That's pretty damn hectic.
Melissa Benoist talks about how they're were stunt and photo doubles on each show, meaning that there were stand-ins for the major characters on each set at all times. If you see Supergirl or Overgirl but don't see her face, it may not be Benoist in that shot.
It's an impressive undertaking to put together four shows with the same actors on all the shows in the middle of filming four active series. A movie takes months to shoot — that's not counting pre- and post-production. Yet these guys put together a four-hour (closer to three with commercials removed) mini-series event in a matter of weeks. And then there's how much time was needed of Benoist and Stephen Amell, who played two major roles. At least Benoist was playing a scene with herself for a while. Amell at one point was on Earth-X as Green Arrow leading that group and on Earth-1 as Dark Arrow leading the Nazis. Whatever your feelings about the show and the crossover itself, what they put together is a remarkable feat.