Yousuf Karsh made the portrait of Churchill you describe.
The story I heard about it is when Churchill walked in to have his portrait made, he had a cigar in his mouth. After setting him up for the portrait, Karsh went over to where Churchill was sitting and, without asking, took the cigar out of his mouth. Hence the look that Karsh captured.
Info on Karsh, including the Churchill portrait:
Correct, Karsh only had a few minutes given to him to take the image so everything had to be done before he arrived (lighting, focus, etc). Churchill came in and at the last moment he pulled the cigar from his mouth and he took the image. This is where a great photographer knows how to get the image they are looking for. Most of Karsh's portraits are posed and I think you would be hard pressed to find an image that wasn't a wonderful portrait.
https://en.m.wikipedia.org/wiki/Yousuf_Karsh
https://karsh.org/
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