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they represent silence - DFW uses them alot in Broom of the System dialogue
I think he also uses them both to let the reader had their own interpretation as well as to place the focus on the answer and the fact that in many cases the answers given reflect the subject's agenda rather than the interviewers and probably would not be much different regardless of what the question had been much like a politician returning to a talking point rather than directly responding to a question.
I haven't gone back and checked but as I recall Wallace used the same technique in Brief Interviews With Hideous Men where the chapters dealing with the interviews aren't even prefaced with a "Q:"