I was looking at fan films on line and came across
www.batinthesun.com. They are currently working on a film called Patient J and the description seems awfully familiar.
"In the basement of Arkham Asylum, a desperate psychologist has secretly set up an interview with the mass-muredering Joker. As the Clown Prince of Crime reflects on the key moments of his life, the interview drifts into a game of manipulation that flirts with life and death."
The gang at BITS may go their own route with this but from the description the first thing that popped into my head was DT.
And I know I spelled Definitive wrong in the subject, but too late to change it.