In de Kooning's case, JFK's restless nature made it difficult for her to paint a portrait as she normally would. Instead of a traditional sitting, she had to prepare several sketches and workshop until she got the final canvas:

http://www.artnews.com/2015/04/10/a-president-seen-from-every-angle-elaine-de-kooning-on-painting-jfk-in-1964/