If you think you've had a hard time juggling schedules, imagine trying to find a time that works for six different tv production schedules to make a crossover happen.

That was the challenge presented to Crisis on Infinite Earths' showrunner Marc Guggenheim roughly a year-and-a-half-ago once he and his partner in crime, Greg Berlanti, were given the green light to proceed on the biggest crossover event in DC history. 

Crisis on Infinite Earths takes place over the course of five nights, crossing over between five shows (six if you count Cress Williams from Black Lightning) and more cameos and guest appearances than we have room to list. 

For Guggenheim this meant working with producers and writers from every show involved in the Arrowverse. Collectively they needed to work out the logistics of taking actors away from their shows for weeks at a time and ensuring that the crossover storyline would not be too big of a departure for any given show's direction.

"One of the most challenging aspects of doing these crossovers is scheduling the actors, and I really wanted a story that sort of had maximum flexibility. And we worked for two weeks refining that scaffolding and coming up with a more detailed set of beats. Then we each retreated to our separate writing staffs and broke out the individual episodes scene-by-scene..." Marc Guggenheim.

To make matters even more complicated, Guggenheim and team had to find a way to get Cress Williams from the set of Black Lightning in Atlanta to Vancouver, where the Arrowverse shows and crossover were being filmed, without disrupting season three of the hit show.

Thankfully all the logistical hurdles are now in the rearview mirror for the Crisis On Infinite Earths team and all that's left to do is just sit back and enjoy!

This year's Arrowverse crossover event begins Sunday, December 8th at 7P on CW26.

You can see more from Marc Guggenheim's interview with Indiewire here.