After what Gambol went through with the Joker, Michael Jai White still hates clowns.
Nestor Carbonell who played Mayor Garcia also played Batmanuel in The Tick.
All-Star Cast: Specifically Christian Bale, Michael Caine, Heath Ledger, Gary Oldman, Aaron Eckhart, and Morgan Freeman. Multi-decade spanning name recognition? Check. Critical acclaim? Check. Awards cache? Check and the mate.
The Joker is often mis-quoted as asking "Do I look like a man with a plan?" when Harvey Dent says that his (Harvey's) disfigurement and the death of Rachel Dawes was part of his (the Joker's) plan, most likely because people associate the rhyming words "man" and "plan". The quote is actually "Do I really look like a guy with a plan?"
"Why so serious?" isn't really the Joker's catchphrase. He only says the phrase in a fierce tone when pretending to be his dad, and the one time he says it as himself he says it almost sarcastically.
Development Gag: When Fox is asked for improvements to the Batsuit, he remarks, "You want to be able to turn your head." Prior to the Dark Knight Trilogy, no physical implementation of the Batsuit allowed the actors to turn their heads.
Directed by Cast Member: Heath Ledger directed and shot the two home videos the Joker sends to the media (both the one where he tortures and kills the fake Batman, then the one where a captive Mike Engel is forced to read a script of the Joker's demands).
Fatal Method Acting: While Heath Ledger is not an example (he died after his parts were done), the stuntman who crashed the Batmobile during pre-production is.
The Other Darrin: Maggie Gyllenhaal as Rachel Dawes in Dark Knight, replacing Katie Holmes from Begins.
Playing Against Type: One of the reasons people were so skeptical about Heath Ledger playing The Joker was that he wasn't known for villainous roles, let alone one of the most psychotic and monstrous characters in fiction, usually playing romantic leads. As we can see from the result, Tropes Are Not Bad.
Reality Subtext: Bruce Wayne requesting a new Batsuit from Fox so that he can turn his head and be more agile, was the result of Christian Bale complaining about these problems with his Batsuit from the first movie.
Recursive Adaptation: Elements of Ledger's Joker were incorporated into the comics, most notably the scars. There may have been an abortive attempt to make him closer still, with someone drawing a line at killing Harley.
While the scene when The Joker blows up the hospital and the explosions suddenly stop, and Heath begins messing around with the detonator until the explosions suddenly start up again was initially credited to his ad-libbing, Christopher Nolan actually wanted "a delayed, staggered explosive effect" in that scene. Considering that explosion scenes are usually filmed with extreme caution, ad-libbing would not have been allowed in this case.
A lot of the Joker's mannerisms were developed when Heath Ledger was getting his makeup done, and would scrunch up his face to create different textures for the makeup. The creepy way in which the Joker sucks on his cheeks was one of these.
Type Casting: Ron Dean playing Mike Wuertz, one of Gordon's detectives. He behaves similarly to his performance as Detective Kelly in The Fugitive.
What Could Have Been: There's some fairly creepy concept art of the Joker before Heath Ledger was cast.