Final answer:
The glum-faced man is a marshal who recognizes Mr Easton as a criminal. He takes him to the smoker's room to confront him, while Mr Easton, unaware that he's been recognized or attempting to manipulate the situation, is smiling.
Step-by-step explanation:
The glum-faced man took Mr Easton to the smoker's room because he was a marshal and had recognized Mr Easton as a criminal he had been tasked to bring to justice. The man's glum expression could have been due to the difficult task ahead of him, whereas Mr Easton's smile might have been because he was under the impression that his identity hadn’t been recognized or because he saw a chance to manipulate the situation to his favor. This causes a sense of irony within the story.
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