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This is an interesting question. I looked it up and found the following,
One night in late September or early October 1885, possibly while he was still revising "Markheim," Stevenson had a dream, and upon wakening had the intuition for two or three scenes that would appear in the story. "In the small hours of one afternoon," says Mrs Stevenson, "I was awakened by cries of horror from Louis. Thinking he had a nightmare, I woke him. He said angrily, 'Why did you wake me? I was dreaming a fine bogey tale.' I had awakened him at the first transformation scene ..."
This excerpt was taken from the source link below.