As insanely busy as I have been lately, I can't help already squinting ahead into the horizon and looking to the next book I'm going to write. The concept, while not directly connected to any of my existing works, is sci-fi/dystopia again. And this, together with some preliminary research I'm doing, makes me think about how different writing fantasy is from writing sci-fi.
In fantasy, you can get away with pretty much anything, as long as it makes sense in the context of your story. Say, your protagonist isn't attacked by dragons because the great bloodthirsty beasts are afraid of the bright yellow color of his waterproof coat. It can and should make a rich, detailed, harmonious tapestry, but ultimately, you're the creator of this world.
In sci-fi, you operate within the limits of our world, be it on planet earth or in space. You use real laws of physics, real geography, and real history, and you have to be sure you don't make any blunders. So you will sometimes find yourself learning all you can about how spaceships work or how to build an igloo. What you suggest in your books must be at least plausible.
The book I have just finished writing, Mountains of Gold, is a historical novel set mainly in Africa. My next book will be quite a jump down the globe, in Antarctic setting (details will be coming later). So I find myself watching documentaries and learning all I can about the ice layers, the currents, the geography, the patterns of light and darkness, and the research stations. If nothing else, it's going to contribute to my education!
Read more about sci-fi vs. fantasy here.