Originally Posted by Mjölnir
It's one way to look at it but lack of empathy really just means that you lack the ability to feel what others feel, it's not a lack of emotions altogether. To me The Joker displays emotion as he enjoys what he does, but he feels nothing for his victims and is perhaps completely unable to do so for anyone.
Exactly. Joker clearly enjoys what he does and that's displayed many times. Some of my favorites include when he bursts out laughing when they knock the Police helicopter out of the air. In fact he's still having a fit of the giggles about it a couple of shots later lol. The torture of the copycat Batman on TV, he is having a ball there. "Why do you dress up like him? Whooo hoo hoo hoo" waving the Batman mask around in front of him. That hilarious mad laughing fit he has when one of his henchmen electrocutes himself on Batman's cowl. When Batman throws him off the Prewitt building he laughs merrily all the way down. He is positively jubilant telling Batman what he's done to Dent. In fact that perfect parting shot of him was him laughing at Batman over that. Joker loves what he does. "I like this job. I like it!!!!".
I particularly love the joy and passion he displays about Batman and how much fun he is for him. We see him annoyed once, when the ferries don't blow each other up. He throws the metal bar he's holding away in anger then whips out the detonator and prepares to blow both ferries up.