Don’t forget to provide your imaginary characters with an imaginary landscape.
I won’t add more helpful hints to this little essay. No–there’s something else I wish to add, which didn’t occur to me seven years ago.
Here’s another aspect to the question. Let’s say you can write believable fantasy. The next question: why do so? Why?
Increasingly I’ve come to view fiction as parables. They’re not factually true, not about real people–but they could be. Parables have to be believable: how else is a parable to teach the lesson it is meant to teach? Our Lord Jesus Christ knew that very well, and made abundant use of parables.
As did, for example, C.S. Lewis in his Chronicles of Narnia. It’s longer than Christ’s parable of The Good Samaritan, but they are obviously related to each other.