taken from an interview w/ Roger Waters in 1979...
TV: So physiologically what stage of the character Pink for the track "Comfortably Numb"?
RW: After "Bring the Boys Back Home" there is a short piece where a tape loop is used; the teachers voice is heard again and you can feel the groupie saying "are you feeling ok" and there's the operator saying, er, "there's a man answering" and there's a new voice introduced at that point and there's somebody knocking on the door saying "come on it's time to go," right, so the idea is that they are coming to take him to the show because he's got to go and perform that night, and they come into the room and they realize something is wrong, and they actually physically bring the doctor in, and "Comfortably Numb" is about his confrontation with the doctor.
TV: So the doctor puts him in such a physiological state that he can actually hit the stage?
RW: Yes, he gives him an injection, in fact it's very specific that song.
TV: "Just a little pinprick"?
RW: Yeah.
TV: "There'll be no more aaaaaaaaaaah!"
RW: Right.
[COMFORTABLY NUMB]
RW:Because they're not interested in any of these problems, all they're interested in is how many people there are and tickets have been sold and the show must go on, at any cost, to anybody. I mean I, personally, have done gigs when I've been very depressed, but I've also done gigs when I've been *extremely* ill, where you wouldn't do any ordinary kind of work.
TV: Because the venue is there and because the act's there...
RW: And they've paid the money and if you cancel a show at short notice, it's expensive.
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the whole article is very good, and he (RW) talks about what every song is about on The Wall. it also has interviews w/ him up until 1999, and as early as 1967.
http://www.pinkfloyd-co.com/band/interviews/rw/rw_frame.html