Here's my copy of Ingres' portrait of Mrs. Gounod. My lady looks more "proper" than his does. His looks like a young, naive, not-entirely-beautiful-and-yet-not-entirely-ugly lady.
Enough of that. While I was doing this I observed that Ingres had incredible control of his pencil and himself. Just about every line that I saw had variable weight. None of the lines was entirely straight and none were of uniform width. This, I think, was a key to his genius in portraying clothing and flesh. I have noticed this trait in other portraits I've studied. For example, rarely is an eyelid of uniform width - generally they are heavier or "thicker" at the ends. ("Thicker" there meaning a heavier shadow.) Such a trick gives the eye a more spherical shape and shows that the eyelid is skin pulled over the ball-shaped eye. As I said, I've seen that elsewhere. But in the Ingres' portraits that I've really looked at, all of his lines are like that. (See Mrs. Gounod's left cheek for the effect.)
From that evidence, I then extrapolate the two observations above. One, that Ingres had incredible control of his pencil and was able to draw that sort of line of varied pressure and twist to points or chisel edges of his pencil. Secondly, I'm guessing that Ingres' had control of himself as well, because it takes a great deal of restraint to not go hog wild and slash fast and hard all over a portrait (especially once it is taking shape.)
One last note - that control over a pencil reminds me of Arthur Guptill's advice in Rendering in Pen and Ink. He has pages of marks that he recommends students practice making so that they can make any sort of mark on command and start and stop it exactly where they want (that's the tricky bit!) I've worked on those exercises over the years and they are tough. But, I've only ever done them in pen - interesting idea to try them in pencil. (BTW, that's a fabulous book. I reread it cover to cover every few years and glance through it often.)