The difference between historical fiction like The Alienist and RPF-fanfic may be that in the first case, the writer and the audience are in tacit agreement that the story and depicted characters are based on real people and real events to a greater or lesser extent, but are clearly speculation-based. Also, historical fiction tends to be about 'safely dead' protagonists, enough removed so that even their direct descendants aren't likely to be reading about their parents doing X when clearly the real parents would have done Y instead.

But surely RPF consumers also agree about the speculation-basis, right? And I never for a moment assumed anything in The Alienist was based on real events, but I did spend too much time wondering about it, and it was one of the factors that prevented me from immersing as I prefer to do.

It doesn't matter to me if X would really have behaved in such a way, the story is just someone's supposition about X. I think a series of stories of Viggo Mortenson wandering around New Zealand getting used to his sword would be funny.

I'm sure most RPF consumers have a good 'this is not really real' filter working, though I think you're more likely to run across people with slender grasps of reality in fandom than elsewhere (based on my extensive experience, not a wildass slam), but I think it's also that RPF consumers/producers may have more unexamined assumptions that doesn't lead them to reminding themselves before writing and/or reading that they are concerning themselves with the not-real activities of real people.

may have more unexamined assumptions

Such as what?

I think there are insane, bad RPF writers out there; just as there are insane, bad gen, het, and slash plain-old-fic writers out there. But I know there are at least a *bunch* of actually-very-clever people who write with the attitude that RPF is media subversion and their opportunity to take an authorial hand to the hegemonically-controlled pop-culture 'scape presented to them.

IJS.

I guess I plain old don't see the subversiveness. And I'm quite uninterested in hypothetical celebrity.

As a wish-fulfillment, as a manipulation, I can see the urge to write. Some. I just don't have the urge to consume.

I'm amusing myself by contemplating Viggo's neighbors in New Zealand chatting to each other about that nice young man down the hall who carries a sword around.

Well, for one 'the past is a different country' -- sf fans who read alternative history are often well-versed in actual history details and get lots of pleasure from picking out where the author has included such details accurately and where they're deliberately departing from the verifiable historical record. But there's also the understanding that if you get into the head of, say, Teddy Roosevelt in 1906, you're dealing with a worldview that basically makes many different assumptions about the world and how it is. He's not a 2002 guy in 1906 clothing, he's a 2002 conception of what a 1906 guy would have thought and felt and acted.

What I think I'm struggling to say is that with historical fiction and alternative speculative fiction -- besides the imprimatur of it being professionally edited and published -- there is a tacit acknowledgement of recognized artifice. Lots of fnords between the lines to show that this is speculation based on real details and people.