Dialogue in Brick

One of the most obvious homages to noir in Brick, was the impeccable use of quick-talking witty language. Although these characters were high school students, in no way did they talk or act like high school students. Spitting out clever comebacks like "Throw one at me if you'd like hash-head. I got all five senses and I slept last night. That puts me six up on the lot of you" is something that you normally wouldn't hear from classmates walking by during passing period.
Though the dialogue may have sounded quite unnatural to your ears, it was also a challenge for the actors. The actors had to undergo some noir research themselves, but they didn't necessarily watch just noir films. They read some of Dashiel Hammet's works, watched Billy Wilder films, and films that "you wouldn't expect to go along with the feel of brick" like Singing in the Rain.  Johnson described the process of attributing the dialogue as "trying to put a tomato into a matchbox; it doesn't fit."  In other words, they kept the uneasiness and unnatural feel that the dialogue brought to the film.
Johnson figured that trying to make the dialogue seem realistic is not going to work out, so might as well keep it the way it sounds. Seeing these teenagers spew out ridiculously fast dialogue is something we aren't used to seeing. It's not exactly our familiar slang that we are all comfortable with.  This what gives the film it's "fresh" sound and makes it different from any other "teen flick."