How do Krakauer's interviews give the book a level of authenticity when it is narrated through a combination of fact and fiction?
I know that by just having the interviews in the book gives insight to Vhris's actual relationships with people and they give authenticity in that way, but as far as Krakauer's writing goes do they impact everything else he says to make it more authentic or are they just a kind of stand alone thing.
I don't know that it does give it any more authenticity. The interview all take place after McCandless is dead, and when we speak of the dead, of our friends and family, especially when it happened recently, we tend to gloss over flaws and transgressions so that they come off as being better people than they were. It makes me want to take the account with a whole shaker of salt, and Krakauer, I think, gives too much credence to those interviews in his interpretation and presentation of McCandless, in part because he has no other choice.
Krakauer's interviews of people who knew McCandless serve several purposes, the first being that it authenticates Kraukauer's own personal feelings towards McCandless. Anyone that hears the basics of the McCandless story immediately thinks that Chris was obviously insane. In the Author's note in the front of the book, Krakauer points out, "Some reader's admired the boy immensely for his courage and noble ideals; others fulminated that he was a reckless idiot, a wacko, a narcissist whol perished out of arrogance and stupidity...". The interviews that Krakauer includes in this book clearly serve to support the idea that Chris was, in fact, "noble"--not one person has anything negative to say about him. He is described as "intelligent" (18) and as a "pretty nice guy" (42). The people interviewed provide evidence that McCandless was not some off-his-rocker young person with a death wish, but that he was a pleasant, smart, amiable young man that was merely holding fast to his convictions and living his life in the way he felt was best.
Krakauer's Into the Wild is an example of creative non-fiction, and as with other works in this genre, narrator reliability is a legitimate concern. You are correct in your evaluation that the interviews serve to reaffirm the authenticity behind Krakauer's story of the life of Chritopher McCandless. This device is important when considering works of creative non-fiction because of the gaps that normally connect points of truth in traditional non-fiction. For example, no one--not even McCandless' family--knows all the events that culminated in Chris' tragic death. Therefore, Krakauer was forced to fill in these spaces with fictional content; this content was likely far more interesting than the actual truth. The interviews are his attempt to ground the text in factual content.