Cat's son Adam is portrayed by Melissa Benoist's (Supergirl) then real-life husband, Blake Jenner. In December 2016 they announced they were getting a divorce, and in December 2017 it was announced that they are now officially divorced.
J'onn J'onzz was born in 1699.
In a 2019 Instagram post, Melissa Benoist revealed that she had previously been a victim of domestic violence, and that an injury to her eye, a torn iris that resulted in a permanently enlarged pupil, was the result of an iPhone being thrown at her. Though he was never mentioned by name, fans who voiced their support for her surmised, based on the time frame of their relationship, that Blake Jenner had been the assailant.
Kara tells J'on, "Dying is a lot easier than getting back up when the world's ended". This is very similar to the line Oliver says in the Crisis on Infinite Earths Crossover, "Dying is the easy part. The real heroes are the ones who have to keep going."
The title of the episode is a references the tagline from the original TV series in the 50's Adventures of Superman (1952).