I ruminate on the challenges of writing a tie-in book that has to fit seamlessly into a series that's still running on the Soap Opera 451 Blog:
The most difficult thing I, personally, found about writing a tie-in was that stories (outside of those stream of consciousness things everyone feels compelled to experiment with in college) require the protagonists to learn something, and then to grow and change as a result of it. (As a writing mentor once said, "Your story needs to be about the most interesting thing that ever happened in this character's life. If this is not the most interesting that ever happened to them, then throw away the story you're currently writing, and write about that.")
Good advice, if you're writing a stand-alone novel. But, as part of a series (not to mention part of a television series), that can be tricky...
Especially if the series you're writing about is going on even as you're composing your story.
With a tie-in, what you want to do is have your character learn something, grow and change... and still be the same person they were when the story started. I visualized the journey as a rope, of which you then tie up the loose ends. Your hero/heroine have to come full circle and end up in the same place they started, so that their adventure may be neatly slipped into the fabric of the main (more important) text - the show, without causing barely a ripple.