Since the two images share a similar pose and lighting, it's possible to compare them for some insights into the subtle choices that Sargent must have been making.
1. Values of the skin tones are simplified and organized.
2. The principle highlights are reserved for the forehead and the nose.
3. The values of the hair are greatly simplified.
4. The mustache is twirled into up-facing points.
5. The face is slightly slimmer.
6. The eyebrows and eyes are drawn with more definite angles.
7. Throughout, there's a visual theme of the heart- or chevron-shape.
People who watched Sargent paint a portrait marveled at the process: "The lightness and certainty of his touch was marvelous to behold. Never was there any painter who could indicate a mouth with more subtlety, with more mobility, or with keener differentiation. As he painted it, the mouth bloomed out of the face, an integral part of it, not, as in the great majority of portraits, painted on it, a separate thing. He showed how much could be expressed in painting the form of the brow, the cheekbones, and the moving muscles around the eyes and mouth, where the character betrayed itself most readily; and under his hands, a head would be an amazing likeness long before he had so much as indicated the features themselves. In fact, it seemed to me the mouth and nose just happened with the modeling of the cheeks, and one eye, living luminous, had been placed in the socket so carefully prepared for it."
Adapted from "John Sargent" by Evan Charteris (New York: Scribner's Sons, 1927).
* from John Singer Sargent, Complete Paintings, Volume 1: The Early Portraits (Vol 1)