Aidan Gallagher (Five) was not allowed to be on set during any gun fight scenes because of child labor laws. Instead a body double was used.
Even though the main events of the first season take place in 2019, no character ever uses a cell phone in any episode. Public pay phones or landlines are used to communicate with other characters.
In season one, Five covers his wall in writing and equations to determine which person to kill in order to stop the apocalypse. He narrows it down to four names which are circled. Milton Green, Aleks Cameron, KC Chavez, and Robert Polko. The last three are names of people who worked in the show's art department.
Gerard Way provided vocals for three songs featured in the show- "Happy Together" and "Hazy Shade of Winter" for Season 1 and "Here Comes The End" for Season 2.