Speaking to Digital Spy at the Monte Carlo Television Festival, Vangsness admitted that she is not sure whether she is supposed to know about the new role but hinted that someone has already been cast, saying she enjoys the unnamed star "a great deal".
Watch Kirsten Vangsness speaking about Criminal Minds below:
Vangness confessed that she felt "miserable" about Brewster's decision to leave the show, but revealed that it is "fantastic" that AJ Cook has returned to the series.
"I love her dearly, and we have a lot of fun," she said. "I'm so happy that she's staying - the girl power continues!"
Vangsness also joked that she loves the "inappropriate" relationship between Garcia and Morgan but admitted that she has no idea how her character will deal with the end of her relationship with Kevin.
"I think they're going to try and then I think she's also going to go frolic and try other things, and there's nothing wrong with that," she said. "I don't know [if she'll be playing the field] but I hope so! Is that bad that I want her to? Play the field, play the field!"
Criminal Minds will return later this year.