The world of literature, whether in comic or pure written form, is perhaps one of the strongest forms of media in existence. While movies, video games, and television shows can bring images to life. A well written work of literature can create for the reader a world so completely surreal and yet real at the same time. Words are only part of the solution though, for any work of fiction to be successful, you need characters. At this point in time, nearly every conflict has been told and portrayed through all the various views and opinions you can give, yet when you add a powerful character to the mix, you have magic.
In comics there is probably no greater villain than The Joker, he perfectly encompasses everything that Batman has fought to secure in Gotham. He is the pure embodiment of chaos and disorder, and yet at the same remains shrouded in mystery. The reader knows everything they need to know about this infamous character, and yet they still want more. There is a magnetism to him that draws the reader into his psychosis, and in some cases his character becomes real.
While it's been awhile since Heath Ledger's tragic death after working on The Dark Knight, I think it's chilling to point out that Jack Nicholson himself claims to have warned Ledger about the role. Nicholson felt that the role is what eventually pushed Ledger over the edge. Citing his time as the villain in the 1989 adaptation of the series. A character this powerful, where being pulled into his psyche becomes part of your day to day life, has apparent impact.
These characters are rare however, but their power draws from the writing behind them. With another Batman series waiting in the wings for Nolan to put a close on take on Batman, it's only a matter of time before another person is cast into the shoes of Batman's greatest villain. The question will surely become if the actor is capable of handling such a role, however my question is whether they will succumb to the character themselves? If another person who plays the Joker winds up deceased shortly before the film's release, we could potentially see a new cursed role.
A character this powerful comes from fantastic writing, and from frightening portrayals, and when they reach this level they garner a life of their own. It truly is a privilege to have characters like The Joker exist in our time. Other characters similar in power exist, but I'm hard pressed to find one, as The Joker embodies everything a nemesis should be; completely opposite of the hero, and willing to stop at nothing to destroy them.