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.

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.