Speculative Fiction, at its heart, is a game of "whatifsmanship." What if Faster Than Light Travel was really achievable? What if Elves and Dwarves and Orcs really existed?
What if Charlize Theron could really find Seth Rogen appealing?
One of the most intriguing forms of fiction then is "Alternative Earth" fiction. Take the Earth as we know it, but change one, core fundamental prospect and see what happens. Harry Turtledove did this masterfully with his classic "The Guns of the South." - what if the Confederates had been granted late 20th century weapons and tactics during the United States Civil War?
A more recent example would be "The Man in the High Castle" - what if the Axis had come out on top during World War II?
Take the world as we know it, and inject a foreign or fantastical element. That's the ticket.
Where Real Meets Unreal
My own "pet" project, Ash Falls, is an Alternative Earth project. It has placed a fictional city in southwestern Oregon overrun with dark urban elements. Vampires, Werewolves, blood-seeking forgotten deities - you get the idea. It is fairly obvious where the "Alternate" element is coming from here, but how about the "Earth" part?
I studied maps and photos of the area in Oregon for weeks. It was critically important to me that readers already having to deal with the idea of bloodsucking demons roaming the streets weren't catapulted out of my story by other elements. The city sits on a river, on the Pacific side of the Oregon mountain range, so it was possible to give the city an almost permanent gloom-shrouded feel simply by the prevailing weather already in the area. I picked a reasonable date for the founding of the town, based on real towns in the area surrounding it. and set up architecture dating back no further than that date. I also made sure I had a large open area around the town for my evil denizens to operate in, should they tire of urban living.
However, the city isn't there in Real Life. So, I was also forced to do some deeper investigation and creation.
Maintain Your Reality
Readers will get hung up on the strangest things. (Investigate the whole "Why didn't the Eagles fly Frodo to the Cracks of Doom" controversy if you don't believe me.) Accordingly, wherever possible, make your reality real. As we mentioned in our Genre Setting: Real World article, your setting had better do its darnedest to feel like a place where your reader would recognize if they were to walk out of their front door and into the setting of your book. There has to be internal logic and consistency or the whole thing falls apart.
Returning to Ash Falls, I had to ask myself - what the heck is this city doing here in the first place? The fact that it isn't easy to enter or leave is covered by a specific fantastical element called the Terlarang, but how the heck are nearly half a million people surviving here? Seeing as how we are set in Southwestern Oregon, I did some research on Oregon and discovered it is the largest woods producer in the United States. It seemed to fit, then, that I developed an economy based on lumber and lumber-associated manufacturing. This simple decision developed literally dozens of story hooks for me, up to and including a millionaire werewolf Timber Baron.
Making the local werewolves rabid environmentalists enabled me to explain away the fact that my city could not have an airport. The city couldn't be entered or left easily, so I had to create a local higher learning institution - Rosedale University. Evaluation of the impact of your changes to reality in an Alternative Earth setting will take you down unexpected paths, and you must be flexible enough to adapt to your core changes or rethink them altogether.
Identify Your Divergence
At the point where you have more or less insulated your world from being broken by your core concept (though you will return to it time and again), you can begin the fun part - figuring out what the mechanics of the "Alternative" in Alternative Earth.
Have you postulated a historical event that turned out differently? Make sure you take the long view here - if you have decided that the Battle of Hastings in 1066 turned out differently, consider what, exactly, that means. Ultimately, it means no British Empire, which means no United States...follow these things as far as you can.
Scientific changes can be just as tricky. If, for example, you go with the popular "the dinosaurs never got wiped out" motif, what does that mean, exactly? Does it mean that humanity never fully developed, ala Harry Harrison's West of Eden? Or, instead, was there somehow room for us to evolve alongside the uber-reptiles into the creatures we are today despite no longer being the apex predator on the food chain?
The questions surrounding your divergence are critically important. You must ask yourself - if this one element was different, what changes would be the consequence? What things would stay as they are?
Returning again to my personal development of Ash Falls, several things had to be addressed. Vampires suck blood for sustenance. After biting the same person three times (as established by Bram Stoker) that person becomes a vampire. Do this enough times, and there is no one left to feed on. I had to sit down and work out the economics of the "vampires to humans ratio" to ensure that it all worked out.
In order to avoid the same problem with werewolves, I had to do away with the whole "being bitten by a werewolf turns you into a werewolf" motif. Instead, werewolves became the result of a magic/genetic change.
We live in a reality of constant observation by security cameras and cell phones. How, then, are the supernatural elements not "outed"? Here I had to go straight to the top of the political structure of my little city and stipulate that the entire police department and city council are under control by warring supernatural elements. Any potential cases involving supernatural elements can be handled at the highest levels. Again, this created story fodder in and of itself. Ash Falls has now developed into a multiple-author, multiple-storyline setting thanks to the adaptations I have had to make to "reality" in order to shoehorn in my supernatural elements.
Don't Place Your Setting Above Your Characters
A dangerous trap lies on the other side of the door leading into your Alternate Earth, however - it is very easy to fall so in love with the setting that you pay more attention to it than you do to the characters.
Again, your story is ultimately about people, not places. Your characters will certainly develop and react based on the environment around them, but they should be the stars of the show, not the meticulously planned and researched Alternate Earth you have created for them to exist inside.
To re-use a very overused metaphor, think "iceberg." About 90% of your setting will never be seen by your reader. I have literally hundreds of pages of notes on Ash Falls, all documenting history and location that will most likely never be introduced to a reader. But it not only makes the setting feel more real, but it also provides a wiki-like source of information for when I need the name of a police commissioner or a historical event that took place in 1962 because my heroic adventurer is trying to unravel a mystery.
The setting must always support your characters, and not the other way around.
What Success Looks Like
Lastly, a little story on an interaction with a fan that will let you know what it looks like when you've done it right.
I was emailed out of the blue by someone who had read Inheritance, the first novel set in Ash Falls. They were wanting to gently correct me over the fact that I did not understand that the Umpqua River was actually two rivers, and did not flow straight to the Pacific the way it does in my book.
Did you get that? They were not unhappy about the vampires and werewolves. They were not unhappy about my description of the climate in Oregon, or my descriptions of landmarks and buildings in (fictional) Ash Falls.
They were upset that I had deliberately changed the course and shape of the Umpqua River.
I've never been to Oregon. I've never seen the river in question in person.
I replied, thanking the person for reading my book. Then I pointed out that the book was fiction, and I had specifically changed the shape of the river to provide a home for my fictional city, Ash Falls.
The city in my Alternate Earth was so real to this reader that they had somehow skipped over the fact that it does not exist and gone right on to thinking that I had mischaracterized the river(s) that ran through it.
And that level of immersion is what we should hope to accomplish in any Alternate Earth we might create.