At left is a detail of Sargent's 1902 portrait of Lord Ribblesdale (Thomas Lister, 4th Baron Ribblesdale) alongside some photos of the subject in similar poses.
Sargent made many subtle accentuations. The head is smaller relative to the figure, making the figure look taller and more regal. Within the face, he made the outer folds over the eyes turn downward more dramatically.
Some of the differences in the folds and lines of the face may be explained by the aging of the subject, but Sargent changed the bone structure and proportions, too. He narrowed the jaw and lengthened the nose. These are all deviations away from the normal, standard face.
To be clear, the painting was done from life, not from photos, but the photo at center, taken in 1884, may have provided a starting point for Sargent's interpretation.
A contemporary cartoonist in Punch emphasized the absurd aspects of Sargent's interpretation. The note says "Regrets...Or Why did I Speculate on Such a Neck Tie? —J.S. Sargent."
Sargent was as much a caricaturist as a copyist, and that's what makes his portraits so memorable.
At the links below, you can see other painting-to-photo comparisons.