Jennifer: Excuse me, Doc Brown. I brought this message back from the future and, well, now it's erased.
Doc: Of course it's erased.
Jennifer: But what does that mean?
Doc: It means your futures haven't been written yet. No one's has. Your future is what ever you make. So make it a good one, the both of you.
Doc: Clara was one in a million. One in a billion. One in a googolplex!
Young Doc: Remember, where you're going, there are no roads.
Marty McFly: You're the doc, Doc.
Doc: This'll shoot the fleas off a dog's back at 500 yards, Tannen! And it's pointed straight at your head!
Clara Clayton: I don't dance very well when my partner has a gun in his hand.
Young Doc: Well, good luck for both of our sakes. See you in the future.
Marty McFly: You mean the past?
Young Doc: Exactly.
Doc: Each detonation will be accompanied by a sudden burst of acceleration. Hopefully, we'll hit 88 mph before the needle gets much past 2000.
Marty McFly: Why, what-what happens after it hits 2000?
Doc: The whole boiler explodes.
Marty McFly: Perfect!
Doc: And in the future, we don't need horses. We have motorized carriages called automobiles.
Saloon Old Timer: If everybody's got one of these auto-whatsits, does anybody walk or run anymore?
Doc: Of course we run. But for recreation. For fun.
Saloon Old Timer: Run for fun? What the hell kind of fun is that?
Doc: Marty, you're going to have to do something about those clothes. You walk around town dressed like that, you're liable to get shot.
Marty McFly: Or hanged.
Doc: What idiot dressed you in that outfit?
Marty McFly: You did.
Marty McFly: How many did he have?
Bartender: Just the one.
Marty McFly: Just the one? Come on, Doc!
Bartender: There's a fella that can't hold his liquor.
Marty McFly: Great Scott!
Doc: I know, this is heavy.
Doc: Marty, I gave you explicit instructions not to come here but to go directly back to 1985.
Marty McFly: I know, Doc. But I had to come.
Doc: But it's good to see ya, Marty.
Engineer: Is this a holdup?
Doc: It's a science experiment!
Marty McFly: Listen, you got a back door to this place?
Bartender: Yeah, it's in the back.
Buford: 8 o'clock Monday, runt. You ain't here, I'll hunt you and shoot you down like a duck.
Gang Member 1: It's dog, Buford. Shoot 'im down like a dog.
Marty: Clara Clayton was supposed to die in the ravine. All the teachers tell the kids a story about a schoolteacher named Clayton who died in the ravine, and all the kids in town remember it because they all have a teacher they'd like to fall in the ravine.
Young Doc: All you have to do is drive the time vehicle directly toward that screen accelerating to 88 miles an hour.
Marty McFly: Wait a minute, Doc. If I drive straight towards the screen, I'm gonna crash into those Indians.
Young Doc: Marty, you're not thinking fourth dimensionally. You'll instantly be transported back into 1885, and those Indians won't even be there.
Answer: He simply has an absurdly low tolerance for alcohol, and whiskey is not a wise choice if this is the case. It helps set up the joke when Marty asks the bartender, "How many has he had?", and he replies by telling Marty, "Just the one", as we are meant to think Doc has been in the bar all night drinking away his sorrows.
Jazetopher