Imagine that one day, after a long afternoon writing your novel, one of your characters appears in your living room. And no, you're not crazy, he's really there. He then tells you about this world where he lives, that one that was supposed to be your creation. But it is as real as yours, as you're soon about to discover.
And then you find yourself in a world that only had place in your imagination. And it is no fairytale, the danger is there and you don't have the comfort of being in a different reality to protect you. All that you made (is it really the term you should be using?) your characters go through, is now happening to you.
The thing is: it's not as simple as crossing realities. Much has been happening to made that happen which you don't know about. Nothing a great adventure shouldn't solve, right?