Writers often need to construct elaborate fantasy worlds in which to set their books, worlds with their own sets of rules and constraints. I’ve done that a time or two myself, but most of the time, I prefer to tweak.
There’s a lot to be said for basing a story in a familiar locale; the research is easy and, most of the time, all the heavy lifting has already been done. In other ways, working that way can be more of a challenge—it’s all detail work, rather than broad sweeping strokes.
As a reader, I enjoy reading a book set in a favorite location or, alternately, I get a kick out of visiting a new location, somewhere I’d previously only read about, and finding it familiar. As a writer, it’s fun creating stories where the lines between reality and fantasy blend. Vampires in San Franciso, fae in Ireland...
Oh, wait, that was supposed to include an unreal element, wasn’t it? Okay, never mind.
There's a line I love from the movie Silverado--yes, I know. I'm showing my age with that reference--but it goes something like this: "Life is what you make of it; if it doesn't fit, you make adjustments." That kind of fits in with what I'm saying. Some stories require more "adjustments" than others. I'm something of a History freak, by which I mean, I'm bad with dates and occasionally more-than-a-little fuzzy on the details, but I love the interconnectedness of events, the cause-and-effect, the way one thing led to another, led to another...leading us here.
When I write stories set in other times--a steampunk past, a dystopian future--I have to play a game of what-if, I have to look for places in History where "my" past and/or future worlds can diverge from reality. I have to determine how that divergence will effect not just the timeline itself, but the worldview of the characters who will be populating this newly tweaked world.
Then I have to figure out how to seamlessly explain what I’ve done to readers. That’s probably the aspect I find most challenging. How do you explain what’s never happened from the POV of characters who aren’t cognizant of the “real” world where these events took place?
For example, I set This Winter Heart in a North America in which the Louisiana Purchase had never taken place. Since it never happened, my characters had no knowledge of it. I could have them discuss the boundaries of all the new countries and territories I’d carved out, I could drop hints as to why things now were the way they were, but the reasons why their Santa Fe differed from my Santa Fe is not anything that would occur to them to wonder about.
I think I tend to err on the side of having my characters say too little about such things, rather than too much. But I wonder what other readers/writers feel about that.
In general, how much do you need to know about why an alternative world is different than our reality? Are you satisfied with hints as to why/when/how things have changed? Do you want to discover the differences as you go, or do you want things spelled out ahead of time?