A Frank discussion of biographies

I recently read Frank Kaplan's excellent biography of Frank Sinatra Frank: The Voice but it got me wondering about the rules for biography writing. 

The book is loaded with examples of things which would seem to be impossible for a biographer to discover through research: people's inner thoughts, exact conversations which happened decades ago, etc. Obviously you expect a certain degree of reconstruction and filling-in-the-blanks and speculations from any biography but at times it almost felt like a novel in its ability to get inside so many of the characters heads. At what point does something cross the line from non-fiction to something like Norman Mailer's Executioner's Song, a novel based on real life events which the author exhaustively researched?

I did find the book a compelling read and hope he's at work on volume 2.