i think the key to the 'scar story' is that he's using the story intimidate someone... because the joker derives pleasure from the fear he sees in the person's eyes. So, its not so much of a sit around a campfire and tell you about it kind of deal... so, if i had to come up with one... it would take place in a bar...
Joker strolls into a honky tonk, steel guitars twanging among the ambient bar babble. He pulls a stool up next to a large man in a cowboy hat drinking a beer.
The cowboy looks over and catches a glimpse of the joker and can't seem to take his eye's off the odd character. The joker turns his head slightly and makes eye contact. The cowboy finds the nerve to scough and looks back across the bar.
Joker: "Say, straaanger? You look familiar."
Cowboy: *laughing* "I'd reckon if I had met a freak like you before, hah, I'd remember."
Joker: "No, no, no... I think, yeah, I remember now... you were in movie.."
Cowboy: "Eh? Movi-"
Joker: "Yeah yeah yeah... what was the name? Uhm... Brokeback Mountain!" *grins*
Livid, the cowboy stands up abruptly and breaks his beer bottle across the edge of the bar with a yell. As he turns, the joker swiftly moves in and and snaps his switchblade, meeting the inside of the cowboy's cheeks as soon as it was released. With a groan and a long pause, the cowboy looks into the jokers eyes and glances at his scars.
"Oh, you like my scars? Funny story. See, I used to be into the, bar scene... like yourself. Years back, a friend and I would go alllll over town hitting joint after joint. BUT, that became... what, whats the word... *licks lip* BORING. So, we started amping up on some of the good stuff, you know, before we'd go out. Well, one night... what we thought was some good stuff... well... turned out to be some bad stuff. REAL bad. *licks lip* Hah, so that night, my friend turned on me... hitting me, kicking... I grabbed a knife and gutted him like a fish. When I woke up, covered in blood... I couldn't believe what I'd done... *shakes head* MY BEST FRIEND! I took the knife to my own face... did all this... made me feel ALOT better. After that, I put down the dope... put down the bottle. But you know what..? Hmm? *licks lips, grits teeth* I just could never seem to put down the KNIFE!"