Jonathan Her interviewer is the author, not necessarily Smith Henderson precisely, but the storyteller. You don't realize that until the last page in the book, where the "interviewer" talks about what kind of guy Pete is, and how he just couldn't go on without finding out if Rachel comes home.
This answer contains spoilers… (view spoiler)[Most of the book I had no idea who Rachel's interviewer was. Some early theories I had are; it was her own social worker, or at times I wondered was it a therapist from a mental institution. In the end I wondered was it even Rachel they were interviewing or was is someone who had witnessed the unraveling of her, or perhaps a group of people who had been with her along the way? In this case the interviewer being a detective of some type. Or, is it possible that it could have been Rachel becoming further and further form herself? She is both the interviewer and the interviewed? To me this seems likely. Anyone agree or have another take? (hide spoiler)]
Sherri F. Glad I took a peek at this b/c I'm in the middle & have been wondering if I missed something (esp. given that I'm listening to it). The feeling might be different between book & audio on this one..esp. given that someone asked if they were same but in audio, the interviewer is male and Rachel/"Rose" is female--BTW love both voices (M is Matthew McC.-like & F=that sweet, southerny narr. fr The Help & Secret Life of Bees). It is hard to tell b/c M will say "Did she receive Pete's gifts/letters?" and F will say "She did" BUT from earlier we already heard it start off w/"Rose," like when a kid goes to friend or counselor and says (at least back in the old days...actually kind of when this was set come to think of it), "my friend" has this problem, let's call her "Rose" type situation. Then in between the M narr/interviewer will say 'What does Rachel/do you think/want/feel etc. and then F does the big sigh (that kids love to give parents) and says 'Roooose not Rachel' but also says Pete not Dad (but some kids do that too, esp. if estranged). I started off thinking it was btwn Pete/Dad & Rachel, then moved to a School Counselor or Therapist theory, BUT then when Rachel say that his job has him helping other families/kids & what abt his own, then I maybe then more Social Worker vs Counselor/Therapist. Just general Storyteller/Author didn't come to mind for me, but only coming up on half-way & it looks like that opinion/theory comes up at the end (according to some at least)?? Still glad I peeked cuz it was distracting me too much worrying if I missed something!