Psychology: Back in the Habit

I was thinking about the role the psychologist should play in the movie, and how she affects the resolution of the film. I want her to have to testify about the kidnapper's mental state during a trial, and I also want the film to be told through a frame narrative which takes place from her point of view.
My vision right now is that she interviews the kidnapper, comes to the conclusion that he/she is aware of the wrongdoings they have committed but only wants to see the world burn- there is no mental health diagnosis, it is just his demeanor. She thinks for this reason, he should be fully responsible for the crime and serve his sentence behind bars. I think she is very disturbed by his thought process, and really wants to see justice served against this vile man.
My research on this character was primarily to find out how a psychologist works within a criminal trial. I wanted to understand the schematics of the impact of a psychologist testimony in a trial, and how they can play a major role in a case.

I plan to start typing a script this week, because I will have so much free time now that Mamma Mia is coming to a close. I want to start filming over spring break, to give me sufficient time to edit and get production shots. I'm so excited to watch this film grow!

RESOURCES
http://criminal-justice.iresearchnet.com/forensic-psychology/psychology-and-law/roles-of-psychologists/
http://criminal-justice.iresearchnet.com/forensic-psychology/psychology-and-law/roles-of-psychologists/
https://www.justcriminallaw.com/criminal-charges-questions/2017/01/27/should-criminal-defendant-testify/
https://www.justice.gov/usao-nj/victim-witness/handbook