In his little-known preface to Treasure Island (which I discovered while answering this question), Robert Louis Stevenson wrote rather persuasively on the possibilities for how a fictional character might be inspired, whether by a stranger encountered once or by a good old friend:
And then I had an idea for John Silver from which I promised myself funds of entertainment: to take an admired friend of mine (whom the reader very likely knows and admires as much as I do), to deprive him of all his finer qualities and higher graces of temperament, to leave him with nothing but his strength, his courage, his quickness, and his magnificent geniality, and to try to express these in terms of the culture of a raw tarpaulin. Such psychical surgery is, I think, a common way of "making character"; perhaps it is, indeed, the only way. We can put in the quaint figure that spoke a hundred words with us yesterday by the wayside; but do we know him? Our friend, with his infinite variety and flexibility, we know ― but can we put him in? Upon the first we must engraft secondary and imaginary qualities, possibly all wrong; from the second, knife in hand, we must cut away and deduct the needless arborescence of his nature; but the trunk and the few branches that remain we may at least be fairly sure of.
As soon as I read the first sentence of this quote, I wondered: who was the friend of Stevenson on whom Long John Silver was based? Silver is easily the most interesting character in Treasure Island - a most "ambiguous rogue", as I once saw him very accurately described - and it would be very interesting to know who inspired him, especially if it was a famous figure "whom the reader very likely knows and admires".