Where do fictional characters come from?

I'm re-reading Joseph Conrad's Nostromo and in the prologue he tells how the idea for the protagonist first came to him. In 1875 he heard about a man who had stolen a boat full of silver during the course of a revolution. Conrad then completely forgot about the story for twenty six or twenty seven years.

Then he chanced to pick up a shabby book; the autobiography of an American sailor. The sailor said that he worked for this man who was now captain of a small boat he had bought with some of the proceeds of his theft. The sailor said that the man had been able to steal the silver only because his employers had trusted him implicitly. Yet the American sailor considered his captain to be a rascal, a small cheat, stupidly ferocious, morose and altogether unworthy of the greatness which this opportunity had thrust upon him.

Conrad wondered whether this would make the basis of a good story but eventually decided that his talents did not run to writing about a rascal who committed a robbery.

'It was only when it dawned upon me,' Conrad writes, 'that the purloiner of the treasure need not necessarily be a confirmed rogue, that he could be even a man of character, an actor and possibly a victim in the changing scenes of a revolution, it was only then that I had the first vision of a twilight country and events flowing from the passions of men short-sighted in good and evil.'

I love this story of how one of the great figures of literature was born. It sent a shiver down my spine.

I began to consider how my characters appear and are changed, the genesis and evolution of them. So many of my favourite characters seem to appear from nowhere, tapping on my keyboard and yelling, 'Write about me, write about me.'

Does this happen to you? I'd love to hear how the characters you write about came into life.