The obvious limitations to writing biography are that the events are established and the characters well defined. However, capturing events and making compelling characters of real people challenges even experienced writers.
Biographies appeal to readers because they want to know more about a person or events.
Once you determine your own motivation, then focus upon the theme of the work. The lives of most individuals are marked by several themes usually marked by a defining moment forcing a decision of some sort. If you are writing about a portion of the person s life, select a period marked by a single theme. If you plan to write a complete biography, organize the work to reflect these defining moments. Before writing a single word of the manuscript, research, research, and then do more research.