The dark night viral marketing campaign.
They used a lot of things for this campaign including - fake campaign for Harvey Dent, fake joker hackers, meetings, and they even had people to protest about what's happening in the fictional city of gotham, and even voice mails from Harvey Dent and the joker when you ring a certain number.
They used the internet, wifi, posters, and word of mouth to convey the marketing campaign and because of all the money they poured into it, the number of people who wanted to watch the film rose exponentially compared to the original amount who wanted to watch it.
The marketing team put in nearly 400 million to promote their film and this could have been a big loss. That's why not many films have tried this kind of marketing before. it is not because the dark knight franchise is unpopular but because they want newer audiences as well as the guaranteed ones.
They made the website called - I believe in Harvey Dent - and people donated money to his campaign and their were fake speeches and interviews.
They previewed the opening bank robbery scene before the screening of Will Smith's film - I am Legend.
They drove dent-mobiles around the city to promote Harvey dent's campaign.
After Heath Ledger's death the film grew in popularity and this was part the reason it sold so well - if fans of heath ledger weren't going to see the film, the would have done after his death, because it was his last ever performance.
They also posted a phone number on the website - I believe in Harvey Dent - and when people rung it they would be received by a Harvey Dent voicemail. Sometimes the Joker would but in.
The I believe in Harvey Dent website was then hacked by the Joker who smeared it with his own slogans.
The Dark knight also drew unwanted publicity due to a shooting in the first screening of the dark knight. This was all over the news.