"No duh" is a product of fear.

Dr. Lilian Thurman: If the sky were to suddenly open up, there would be no law, there would be no rule. There would only be you and your memories.

Dr. Lilian Thurman: If this world were to end, there would only be you... and him... and no one else.

Eddie: That damn airline better not fuck us on the shingle match.

Prof. Kenneth Monnitoff: And did you stop and think that maybe infants need darkness? That maybe darkness is part of their natural development?

Roberta Sparrow/Grandma Death: Every living creature on this earth dies alone. [Note: Is said at a nearly inaudible level, later repeated by Donnie ]

Ronald Fisher: Go back to China, bitch.

Sherita Chen: Chut up!

Donnie Darko: Why do you wear that stupid bunny suit?
Frank: Why are you wearing that stupid man suit?

Donnie: [taking a cigarette at the bus stop] What will happen if you tell mom about this?
Samantha Darko: You'll put Ariel in the garbage disposal.
Donnie: Goddamn right I will.
Sean Smith: Good shit, eh?
Donnie: Dude, it's a fucking cigarette.

Donnie: Do you want your sister to lose weight? Tell her to get off the couch, stop eating twinkies and maybe go out for field hockey. You know what? No one ever knows what they want to be when they grow up. You know it takes a little, little while to find that out, right, Jim? And you... yeah, you. Sick of some jerk shoving your head down the toilet? Well, you know what? Maybe... you should lift some weights, or uh, take a karate lesson and the next time he tries to do it, you kick him in the balls.
Jim Cunningham: Son... DO YOU SEE THIS? This is an Anger Prisoner. A textbook example. DO YOU SEE THE FEAR, PEOPLE? This boy is scared to death of the truth. Son, it breaks my heart to say this, but I believe you are a very troubled and confused young man. I believe you are searching for the answers in all the wrong places...
Donnie: You're right, actually. I am pretty- I'm, I'm pretty troubled and I'm, I'm pretty confused. But I. . .and I'm afraid. Really, really afraid. Really afraid. But I... I... I think you're the fucking Antichrist.

Donnie: Frank, when's this gonna stop?
Frank: You should already know that.

Donnie: How do you do that?
Frank: I can do anything I want. And so can you.

Donnie: How's it feel to have a wacko for a son?
Rose Darko: It feels wonderful.

Donnie: I made a new friend today.
Dr. Lilian Thurman: Real or imaginary?
Donnie: Imaginary.