I’ve answered versions of this question a couple times, but I keep forgetting one very important influence:
Eric Bogosian.
Go watch Oliver Stone’s Talk Radio. Bogosian spends two hours steadily descending into madness and despair behind a microphone, and I’ve watched every embittered, explosive minute of it, dozens of times over.
The big thing for me is the synergy between Bogosian’s writing and his voice. Remember the old thing Harrison Ford supposedly told George Lucas about his Star Wars script, how “you can type this shit, but you sure can’t say it”? Every syllable of Eric Bogosian’s writing is intentional, and designed for the ear more than the eye.
Also, his rants are truly epic, and very clearly influenced the way Bill Lawrence wrote Perry Cox on Scrubs. (Bogosian was Cox’s therapist in one episode, and John McGinley plays an audio engineer in Talk Radio. Everything is connected.)
See also: SubUrbia (written by Bogosian), Sex, Drugs, Rock & Roll (his one-man show from ‘91)