Thirteen Days

Thirteen Days was the title of a book written by (or ghost written for) 
RFK about the Cuban Missile Crisis. It was a source material for the made 
for TV program The Missiles of October which was essentially a televised 
stage play. Several decades later it was made into a major motion picture 
of the same name as the book. However there was something curious about 
the movie which I still don't understand. Kenny O'Donnell was barely 
mentioned in the book and had only one scene in the Missiles of October. 
In the movie Thirteen Days, he was made the central character played by 
Kevin Costner. Essentially the crisis played out through his eyes. I've 
always wondered why the producers chose to go in that direction when it 
seemed from RFK's book that he was barely involved in the management of 
the crisis. Was there other source material that indicated O'Donnell's 
involvement was much greater than what it appeared in the book? If anyone 
has an information as to why the movie expanded O'Donnell's role beyond 
what the book indicated, I would appreciate it.