Though many have been related, a definitive backstory has never been established for the Joker in the comics, and his real name has never been confirmed. He has been portrayed as lying so often about his former life that he himself is confused as to what actually happened.
The most widely cited backstory can be seen in The Killing Joke... Terrified, the engineer leaps over a rail and plummets into a vat of chemicals. When he surfaces in the nearby reservoir, he removes the hood and sees his reflection: bleached chalk-white skin, ruby-red lips, and green hair. These events, coupled with his other misfortunes that day, drive the engineer completely insane, resulting in the birth of the Joker.
The first origin account, Detective Comics #168 (February 1951), revealed that... After committing the theft, which Batman thwarts, Red Hood falls into a vat of chemical waste. He emerges with bleached white skin, red lips, green hair, and a permanent grin.
The second arc of Batman Confidential (#7-12)... (states that he) falls into an empty vat. Wild gunfire punctures the chemical tanks above him, and the resultant flood of toxins alters his appearance to that of the Joker.
I just think some fanservice was in order here. Of course it didn't make the movie bad or anything, but I feel as if it would've been nice not to do things like show his makeup rubbing off in the interrogation scene or, worse, show him completely without it, as a nod to the original comic.I don't see how it could have been done any other way without looking ridiculous. The make-up gave him a very gritty look that went very well with the ambiance of the movie.