Wikipedia:
[David] Mamet’s style of writing dialogue, marked by a cynical, street-smart edge, precisely crafted for effect, is so distinctive that it came to be called Mamet speak. He often uses italics and quotation marks to highlight particular words and to draw attention to his characters’ frequent manipulation and deceitful use of language. His characters frequently interrupt one another, their sentences trail off unfinished, and their dialogue overlaps. Mamet himself has criticized his (and other writers’) tendency to write “pretty” at the expense of sound, logical plots.
When asked how he developed his style for writing dialogue, Mamet said, “In my family, in the days prior to television, we liked to wile away the evenings by making ourselves miserable, based solely on our ability to speak the language viciously. That’s probably where my ability was honed.”
One classic instance of Mamet’s dialogue style can be found in Glengarry Glen Ross, in which two down-on-their-luck real estate salesmen are considering breaking into their employer’s office to steal a list of good sales leads. George Aaronow and Dave Moss finagle the meaning of “talk” and “speak,” steeped in fraudulent connivance of the language and meaning:
Moss No. What do you mean? Have I talked to him about this [Pause]
Aaronow Yes. I mean are you actually talking about this, or are we just…
Moss No, we’re just…
Aaronow We’re just “talking” about it.
Moss We’re just speaking about it. [Pause] As an idea.
Aaronow As an idea.
Moss Yes.
Aaronow We’re not actually talking about it.
Moss No.
Aaronow Talking about it as a…
Moss No.
Aaronow As a robbery.
Moss As a “robbery”? No.