Analyzing Fiction

I've wondered throughout the semester what role an author's life play in analyzing fiction or nonfiction. We've seen how authors can almost be seen as "ghosts" through time in that they are able to step out and see things nobody else can see. However, for example, John Winthrop, author of the sermon City Upon a Hill, was a lawyer before delivering his sermon. He uses legal language throughout the sermon. Walt Whitman, an assumed homosexual, reveals hints of his homosexuality in his poetry specifically in Leaves of Grass. How much does an author's life or even life experiences shape the literature they write? How much of it is something we subconciously seek? Are author's truly "ghosts" through time or are they humans like the rest of us?