Let me start by saying that on the whole, I enjoyed this film a great deal. All of my constructive criticisms are on smaller issues, but there's nothing wrong with the big picture. First off, I love the way the Joker's war paint evolves as he gradually dominates the sessions. In terms of matching Heath's portrayal, this was a generally faithful performance. The voice had strong moments and weak moments, but Chris Notarile had the body language and subtle movements down 100%. One of my favorite little moments was the Joker's grunt/sigh when the first session ended. It was little things like that which seemed so genuine that really enhanced the performance and made it believable.
I did have some issues with Harley, however. Again, the actress did fine on the whole. She was absolutely great before Harley begins to lose control, giving a convincing "professional Harley" performance. However, once the Joker starts to get through to her, I feel like the shifts in her persona are too abrupt and on-the-nose. I think my biggest issue with Harley was the Arleen Sorkin voice at the end. One thing I never bought about the character in the show and the comics is that a bright, ambitious psychiatrist becomes a goofy, bimbo-voiced buffoon. I would have been interested to hear a new, realistic take on the voice, just as Heath Ledger didn't talk like Mark Hammill. I've always wanted to see a Harley Quinn who is as darkly demented and actually as scary as The Joker, and I think you could pull it off if you follow this up with another short.

There are a lot of cheap Dark Knight fan films which often unintentionally border on parodies, but "Call Me Mr. J" is certainly not among them. This is a diamond in the rough of the "joker fan film" youtube search. Great job, and I look forward to seeing more from all involved!