The NPR interviews might be worth looking at, as they talk about the background of the interview, specifically, that Frost was considered a lightweight, there was the desire to put Nixon on the trial he escaped from having, and Nixon was hoping for a sympathetic ear to give his side of the story.
no subject
The interview with JIm Reston might be a good place to start.
http://www.npr.org/templates/story/story.php?storyId=98995421&ft=1&f=1012