The most difficult part for me is when the sitter wants a full figure. Often the hands are a difficult part. I find that facial expressions are not difficult. Changes and cancellations, delays, and what have you, can put one's agenda into chaos. I have a schedule, they have a schedule, sometimes they don't mesh.
Many artists rely on photographs or preliminary sketches prior to the actual painting.
Mrs. Giffuni finds these unnecessary, unless the portrait is to be a surprise, or the subject is deceased. One of her more successful portraits, now hanging in City Hall, of New York City's former mayor Vincent Impelleteri, she painted in oils from a small snapshot. She had to do a full figure, a life-size portrait of the man and was given a photograph of just the head to work with. To do the full figure, she had men of his stature sit for her. As with Mrs. Giffuni, about one-third of Jeff Webb's entire work is devoted to portraits, meaning portrait commissions.