Don't expect anything like deep theoretical analysis, that's really not my thing. I'm a writer, and although I do write on a very conscious level, literary theory certainly isn't my forte.
But the fact that I've written 2 Lumione fics in a very short time, and am now giving birth to a third, got me thinking.
What makes me write 3 fics with the same protagonists? Their names and certain character traits are of course the same, but Hermione is a very different person in Universal Gravitation, Pureblood Courtship and the as of yet untitled fic. The difference is especially notable in no.s 2 and 3 (as you will soon be able to see for yourselves). In P.C. she is of course still very young, whereas in Untitled she's about 26 and in U.G. she's 23. So I can't be sure whether P.C. Hermione might not eventually become like her older selves in the other two fics.
It's more obvious with Lucius, whose age varies between about 50 and 57 in the three stories. At that age - whether wizard or Muggle - people's characters are more or less settled. And still, the three Luciuses are quite different.
Now there are, as I said before, certain fundamental character traits I would never change. Those do, of course, vary from author to author. I, e.g., could never write a completely dark, sadisitic, torturing Lucius coming together with victim!Hermione. Impossible, it just doesn't work that way for me. I see Lucius as a man who in his youth was very similar to Draco as described in the books: snotty, arrogant, way too sure of himself without being self-assured, a young man with a powerful father whose shadow is both burden and hiding place. He probably joined Voldie while still at that age and stage, found out later that he'd bitten off far more than he could chew, and was very glad indeed when the Dark Lord kicked the bucket. Or so he thought. In my personal view, Lucius is very much a family man - those who've read Sybil's Oracle will know that. The family is holy, he is the patriarch, and woe on whomever dares touch his nearest and dearest. I think he was less than happy when Voldie returned. Going to Azkaban might have stoked his hate of the wizarding society as it is and brought him closer to Voldie, but then his Master made the big mistake of touching his son. That's when all bonds were severed, once and for all.
This is my fundament, on which all three Lucius characters are built. The rest is more or less variable.
Same for Hermione: She's clever, she's bossy, she's fiercely protective of those she loves. I don't see her as a teacher, e.g., because she doesn't have much patience with the less gifted. She's also devious and, if need be, totally devoid of scruples (Umbridge). These traits may appear a bit stronger or weaker, according to age and experience, but that's my fundamental Hermione.
The real fun, however, seems to be in trying how many different buildings I can construct on the very same fundament. Buildings that won't crumble, that is.
Funny enough, the character differences between Lucius in P.C. and Lucius in U.G. only emerged while I was writing. P.C. for example had originally been planned as a story around, not leading up to, the 'Vile rapist!' scene. There was meant to be a bit of conflict, Molly and Fleur hadn't been part of the original planning. But then Lucius decided to be playful, and Hermione wanted to be young and refreshingly bossy, and the two Kneazles wanted to form a counterpoint to all the action going on between their humans.
The Muse suddenly appeared in U.G., because, very much like Shiv, I hate Author's Notes giving long-winded explanations to the reader but felt that there had to be something in addition to the storyline. I wanted a harsher Lucius for U.G., but he turned out to be surprsingly soft. (Ahem.)
What I'd be really interested in finding out - and hopefully some day I will - is how things would be if I wrote my own original fiction. Different, I guess. Because there wouldn't be various possible biographies for my protatgonists (as is the case in HP fan fiction), but only one I would have to work out for them, which as good as excludes tinkering with their characters further into the story. Take one of my all-time favourite characters, Lord Peter Wimsey. He is a very fleshed-out character, with a rather detailed biography. Had D.L. Sayers wanted to change him, she would have had to do so by some Deus-ex-Machina manoeuvre, which in itself wouldn't be very believable. Or send him off to two years of psychoanalysis, in order that he may get over his shell shock trauma. But then he wouldn't be himself anymore, which kind of bars that avenue. And, given his detailed biography and characterization, there is almost no room for making him react differently in similar situations.
And, to tell the truth, I suspect that the offer of ready-made but not very fleshed-out characters, together with a ready-made universe you don't have to invent, research and describe, might be one of the main reasons why I'm writing fan fiction and not original fiction.
Not that this fact makes me wish for JKR to kill off Lucius, Snape or Hermione, mind you!