Final answer:
The sentence 'He stood still and his heart quickened with worry' best complements the anxious tone of paragraph 6, resonating with the thematic elements present throughout the provided text excerpts.
Step-by-step explanation:
The best sentence to add descriptive detail to paragraph 6, which follows the theme of apprehension and uncertainty, would be: He stood still and his heart quickened with worry. This choice aligns with Thornton's depicted emotions and the overall tense atmosphere as brought out by the textual evidence.
Examples from the provided texts show characters grappling with fear, anxiety, or moments of tension, supporting this choice as the most fitting. For instance, the mention of a man contemplating meeting death with dignity highlights a profound unease consistent with Thornton's situation.