With the first part of 'Abbott Elementary' and 'It's Always Sunny in Philadelphia's crossover airing Wednesday on ABC, Kaitlin Olson has some tips.

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 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.

Abbott Elementary creator and star Quinta Brunson has issued an important advisory to fans about season four’s midseason premiere, which featured a special crossover with another hit ABC sitcom, It’s Always Sunny in Philadelphia.