I’m not an actor, but I have to think that playing a real person is much more difficult that playing a fictional one.
In most cases , an actor can tap into the motivations and reactions of a character to create the way he speaks and moves.
In the case of a real person, he or she may be well-known enough that the audience for the project is familiar with the appearance and demeanor of the subject.
Makeup artists also have a tougher jobs working on biographical films. Below are a few examples of how well the makeup artist recreations can stand up to comparisons to a subjects actual appearance.