I totally believe this. On the taped call where she is begging Lee for Tony's phone #, "Christina" says something about how people are questioning Casey's lack of emotion and Casey starts saying how she has to be strong, blah blah blah and her voice starts cracking. It's like the lightbulb went off, "Oh, this is where I am supposed to act sad."
Casey is about to make some new imaginary friends. Gertie the Guard and Jenny the Jailer.