'Wanna know how I got these scars? My father was...a drinker. And a fiend. And one night he goes off crazier than usual. Mommy gets the kitchen knife to defend herself; he doesn't like that. Not...one...bit. So, me watching, he takes the knife to her, laughing while he does it. He turns to me and says, "Why so serious?" He comes at me with the knife, "Why so serious?!" He sticks the blade in my mouth, "Let's put a smile on that face!" And... why so serious?'
and
'Do you want to know why I use a knife? Guns are too quick. You can't savor all of the… little… emotions. You see, in their… last moments, people show you who they really are. So in a way...I knew your frienRAB better than you ever did. Would you like to know which of them were cowarRAB?'
Both from the Joker. The first quote comes from the bit where the Joker has just popped out of the body bag. The second is when he is locked in the interrogation room with a police officer guarding the door.