It was originally Terry Crews who started saying "NINE NINE!" on set to help motivate other cast members. The writers liked this very much so they decided to add it into the series.
Andy Samberg (Peralta) and Chelsea Peretti (Linetti) went to the same elementary school, just as their characters had in the series.
After shooting each scene a few times verbatim, the actors then shoot a few more when they improvise to make the scene funnier if possible. This method is also used on Parks and Recreation (2009), co-created by this show's co-creator Michael Schur.
FOX cancelled the show in May 2018, but NBC picked it up for a thirteen episode sixth season. Shortly after the viewers started praising NBC for this, five episodes were added to the initial thirteen-episode deal.
In an early episode, Terry Jeffords (Terry Crews) has to sketch the description of a criminal, due to the normal sketch artist calling in sick. This actually happened to Terry Crews in real life, where he got his first job in the local news station as a sketch artist due to the regular artist calling in sick.