I guess David McCullough did his research by reading letters from George Washington, letters from his wife and family, books that Washington wrote, military memos and any kind of correspondence in order to get some kind of personality profile on George Washington.
It's interesting, we may have paintings on those early patriots, but, Washington has a French sculpture come and make a plaster mask of his face, and that is the closest thing that we can come to of knowing what Washington looked like.
In David's book, he described that Washington had grayish hazel eyes, a stature that commanded respect, yes, Washington was firm, but, he also was a fair and honest kind of leader. ( it sounds like he was a no nonsense kind of man ).
I don't know were it read it from before, but, I read a story of Washington while he was in Valley Forge.
On one cold winter morning, it was snowing, and Washington was about to sit down and eat his breakfast, then he looked out the window and saw one of his troops doing his guard duty rounds and Washington got up from the table and went out the front door and yelled out to this young grunt and asked him if he had his breakfast this morning.
Then Washington ( the Commander in Chief ) told the young trooper to come inside and have breakfast and coffee, and while the young trooper ate, George Washington went out side and did the troopers guard duty..... imagine that ? the Commander in Chief doing the guard duty rounds for this young trooper while the trooper ate George Washington's breakfast.
That is the kind of leaders we need today.
George Washington commanded respect not because he demanded it, but, for the person George Washington was.