The captain/seaman had the look of a man who sees a ghost, or the evil one or something worse. What does this reaction tell you about the captain/seaman and does it make you understand his ways ever since he arrived at the inn?

Are you referring to Billy Bones? Billy Bones is the first pirate that Jim meets in the book - his appearance (ragged, scarred, ponytail, and a cut on his check) signals the end of tranquil at the inn. Despite his drinking (especially rum) and singing a song that is clearly a pirate tune, his seeking out the Admiral Benbow inn is a sign that he does not want to be captured. Although Billy Bones demonstrates this dubious behavior, Jim is not afraid of him and even enjoys the excitement that the seaman brings to the otherwise isolated island. In return, Billy Bones is kind to the young boy. Billy Bones is a precursor to one side of Long John Silver's personality, the kind, gentle, parental side that is present when he aligns with the "good" men. Although he is blustery, beneath it all he is good-hearted. Jim is genuinely sad when he passes away at the end of the first part of the book, from a stroke.