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Source: https://writers.stackexchange.com/a/27044
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

Your question is too broad. I'll narrow it down for my answer.

**Procedure**

In the case of coauthored biographies, the coauthor typically interviews the primary author (and other persons), assembles a text from these interviews, and works in feedback from the primary author. That is why such a biography "sounds" like the primary author: it contains what he _said_.

Typically the primary author in a coauthored biography has little or no writing experience (Cary Elwes, for example, never wrote anything that I could find), while the coauthor is usually a seasoned professional writer (in this case "[award-winning journalist and bestselling author, Joe Layden has written more than thirty books](https://www.harpercollins.com/cr-101358/joe-layden)").

**Proportion of Work**

From the typical procedure, the typical proportion of work the coauthor of a biography does follows implicitly: the primary author does all the talking, the coauthor does all the writing.

So when Elwes says in interviews that he 'wrote' the book, what he means is that Layden wrote it: "[Elwes was interviewed by co-author Joe Layden in the writing process](http://www.deseretnews.com/article/865617108/New-book-from-Cary-Elwes-gives-fresh-view-on-The-Princess-Bride.html)".

**Other Kinds of Non-Fiction**

are different.

Original score: 3