According to the 1890's Society, William Rothenstein, and a few other artists, were commissioned to do a number of drawings of famous London celebrities by Grant Richards who pulled them all together for an independently published booked called "English Portraits: A Series of Lithographed Drawings". All told there were only some 750 copies printed. 

Each drawing accompanied an insight or biographical-comment submitted by yet another famous celebrity about the person featured. It was kind of a Who's Who meets People magazine type concept and Sargent was just one of a number of celebrities that William drew.

It's funny, at the 1890's website, they describe Sargent like this: "Rothenstein's Sargent appears almost to have been caught unawares and captured against his will by his fellow 'lion hunter' [fellow portrait artist], with a cigarette still dangling from his lips and an expression of wariness and defensiveness in his eyes."   

Is it that Sargent is "wary," or is it that Sargent himself may have been standing and concentrating as he himself was sketching Rothenstein at the same time?
(1890's Society)
 
 

John Singer Sargent

President Woodrow Wilson
1897

 Notes
 

• See more on William Rothenstein