Criminal Minds was no stranger to cast turnover, losing key protagonists and going through a number of seasons in which team members seemed to arrive and leave within the same year. Many departures became well known, including the dismissal of Thomas Gibson after an alleged on-set altercation and the early loss of Mandy Patinkin over the show's content. One that was nearly as dramatic, however, was the departure and return of A.J. Cook's Jennifer Jareau, better known as JJ.
Source：https://www.cbr.com/criminal-minds-why-aj-cook-jennifer-jareau-left-returned/
