Determine whether there's a viable market for this story, making it worth a producer's time and financial investment. Your Aunt Caroline, for instance, may be the loveliest person on the planet but if the only thing she ever did was make great fruitcakes and play with her cat, audiences probably won't rush into theaters. For a biography to work successfully as a commercial film, the main character should be either an ordinary person who survives extraordinary hardship, or a gifted individual forced to function in an ordinary environment.
Identify whether there's sufficient, sustainable conflict to hold viewer interest. While a PBS documentary can leisurely and chronologically spin a tale from cradle to grave, a feature film has to hit the ground running with a significant crisis the lead character must resolve. Unlike real life which is a succession of diverse problems, a screenplay focuses on a single obstacle. Most screenplays begin by introducing that obstacle within the first 10 pages.
Assess whether there are strong visual elements to this biography by making a list of events that involve action. Film is a visual medium. If your biography adaptation is just about talking heads in limited locations, your audience will get restless and bored. Consider the message you want to leave with your audience and whether it's primarily entertainment, educational or inspirational in scope.
Find out if the story rights are available for adaptation. Celebrities, for instance, are entitled to the same privacy rights as anyone else even though they lead very public lives. If the individual is still alive or has heirs with a vested interest in how she's depicted, you need to get written permission to proceed. If it's someone famous who has been long dead--George Washington, for example--you have more latitude in advancing your own spin. What you can't do, however, is take a biography written by a contemporary author and adapt your screenplay from that particular published work.
Research whether your biography subject has been depicted in the medium of film before. Conduct quick research on the Internet Movie Database: using the pull-down menu, do a character search. Prior to penning your own script, it would be useful to rent DVDs of existing films and see how the biography has been handled by other writers.
Study the craft of screenwriting by studying how-to texts such as those published by Michael Wiese Productions as well as downloading biopic scripts from sources such as Simply Scripts. (See Resources.) Screenwriting is both an art and a skill--an art because it calls for originality and a fresh slant, and a skill because the basics of good writing and the rules of industry formatting must be observed.