"I don't, I don't want to kill you! What would I do without you? Go back to ripping off mob dealers? No, no, NO! No. You... you... complete me."
The Joker says this to Batman in their final confrontation, declaring that they are more the same than Batman could know. The Joker tells Batman that he does not want to kill him because he is such a worthy adversary.
"You either die a hero or you live long enough to see yourself become the villain."
Dent says this during a dinner with Rachel, Bruce, and Bruce's date. He suggests that everyone is corruptible and no one is morally superior; rather heroes are just villains who haven't become corrupted yet.
"...Because sometimes truth isn't good enough. Sometimes people deserve more."
At the end, after Harvey Dent's death, Batman decides to take the fall for Harvey Dent, and says this. It suggests that he sees it as his duty to shoulder the blame for Two-Face's violence, as it is for the good of the city.
“Some men aren't looking for anything logical, like money. They can't be bought, bullied, reasoned, or negotiated with. Some men just want to watch the world burn.”
This quote highlights the theme of chaos within the film and within the character of the Joker. When Batman expresses his misgivings about pursuing the Joker to his butler, Alfred offers this nugget of wisdom to suggest that some villains are just evil, and do not have clear motives.
“Nobody panics when things go 'according to plan.' Even if the plan is horrifying! If, tomorrow, I tell the press that, like, a gang banger will get shot, or a truckload of soldiers will be blown up, nobody panics, because it's all 'part of the plan.' But when I say that one little old mayor will die, well then everyone loses their minds!”
Joker says this to Batman in their final confrontation. He suggests that people are rule-bound, and that he can make ordinary citizens do horrible things if he merely presents their situation as a game. The Joker states that he wants to create chaos by making it seem like order.
"You wanna know how I got these scars?"
The Joker asks this question multiple times throughout the film, usually before killing someone with his knife, and each time, he offers a different explanation for his scars. This shows his inconsistent and unpredictable nature.
"Carbon fiber, .28 caliber, made in China. If you want to kill a public servant, Mr. Maroni, I recommend you buy American."
When we first meet Harvey, he is cross-examining a fall guy in Maroni's mob, and the man tries to shoot him with a gun. As Harvey grabs the gun, he says this to Maroni, to suggest that Maroni screwed up in trying to assassinate him.
"I believe that whatever doesn't kill you, simply makes you... stranger."
This is one of the first things the Joker says, after he robs the mob bank in the first scene. It is a play on the phrase, "Whatever doesn't kill you makes you stronger"; in the Joker's world, adversities make one a freak. The line is an eerie allusion to his outrageous and off-putting clown makeup.
"He's the hero Gotham deserves, but not the one it needs right now. So we'll hunt him. Because he can take it. Because he's not our hero. He's a silent guardian. A watchful protector."
Gordon says this at the end of the film to his young son, who suggests that Batman is a hero. Gordon clarifies that the people of Gotham are not ready for a hero like Batman, that he needs to be more discreet and potentially vilified by the citizens before he can be embraced.
"I don't have an answer."
When Harvey initially proposes, this is Rachel's response, and signals to him that she is maybe in love with someone else, or ambivalent about their relationship.
