As executive producer Marc Guggenheim says in the video below, no one has ever tried to crossover four shows like this before. Even last year's Arrowverse crossover, Invasion, wasn't really a full crossover. Only the last few minutes of Supergirl was involved and the Arrow episode was just that, the 100th episode of Arrow and didn't really feel like this one does. The Crisis on Earth-X event feels like a four hour movies so far that just happens to take place in Supergirl, Arrow, Flash and Legends of Tomorrow. It even has it's own opening credits… but no voice over.
The video below features writers and actors from the shows talking about the event and how it's different. How everyone had to pull together to make it work and how at times they'd have up to 22 heroes on the screen at the same time. Its quite the feat to pull together four different writer's rooms to make a cohesive story that fits in with each series season narrative. They're addressing the state of relationships on each show and moving those relationships forward during the event. But there has been no discussions about the big bads of each series. No mention of Mallus, The Thinker, Reign or the crew Cayden James is pulling together. There was a joke about Damien Darhk though.
The second half of Crisis on Earth-X airs tonight starting at 8 PM on the CW.