The star of the FX series also admits he was initially hesitant about the crossover concept as 'It's Always Sunny in Philadelphia' heads into its 17th season.

Here's a look at our real-time thoughts on tonight's crossover between FXX's It's Always Sunny in Philadelphia and ABC's Abbott Elementary.

Originally, Danny DeVito and William Sanford Davis' characters were supposed to know each other from high school, with each having gone after the same girl.

When he found out about the cameras that filmed all of Abbott Elementary’s happenings, he immediately declined to be part of the documentary. Then, throughout the episode, you could see him occasionally evading the cameras and running away.

The Abbott Elementary & It's Always Sunny In Philadelphia crossover had some ridiculous rivalries, but Charlie got a surprisingly uplifting subplot.

The cast of "It's Always Sunny in Philadelphia" will appeared on "Abbott Elementary." Here's how to watch, when it airs and what to know about the crossover.

Here's some background and inside jokes to help fill in some of the gaps for viewers who don't watch both Philly-based shows.