Where Do Fictional Characters Come From?

I have enjoyed reading Alan Hollinghurst's new novel The Stranger's Child, which centres around a First World War poet called Cecil Valance and I notice that some people have been having fun trying to identify "the original of" Cecil, arguing, for example that he is "based on" Rupert Brooke.   This in spite of the fact that Brooke is mentioned in the novel alongside Cecil Valance.  I found myself playing the game too, seeing Julian Grenfell and maybe a bit of Charles Hamilton Sorley in this character. And then I reminded myself that this is fiction.   That's right: invention, imagination, creation.   Novelists, even conventionally realistic ones, are not newspaper reporters (perhaps a bad analogy just now but you know what I mean) but they may well build their characters out of the raw material of people they have known and experienced.  In an autobiographical novel the relationship one supposes is clear enough but mostly characters are amalgams of perhaps three or four people or they are pure invention.  In so far as we observe the mantra of the creative writing classes – Write About What You Know – then we will draw on actual experience, but surely what matters is the significance and meaning of the character in the novel's overall aesthetic structure.  "The only sure truth about characters in prose fiction," wrote Susan Sontag, "is that they are, in Henry James' phrase, 'a compositional resource'."  The more we emphasise the realism of the novel over its other elements, the more we will think these detective games matter.