IAMTW Member Karen Traviss wrote this article about writing STAR WARS tie-ins back in March 2006 for the Emerald City website...but her insights are as relevant today as tey were three years ago. Here's an excerpt:
From the writer’s point of view, media tie-in fiction is harder and actually requires more original thought and creative effort than creator-copyright — provided you treat it and the readership with the respect both deserve.
The only people who believe it isn’t harder and requires less creativity are those who haven’t actually done it, or perhaps those who think they can coast on tie-ins and just phone it in. But readers can spot lack of effort a mile away in thick fog, believe me, and they’ll never let the writer forget it.
The craft involved is more difficult. Having a foot in both camps, I can tell you that it’s very easy to create your own characters and story arcs; the only holes you find yourself in are those of your own making, and if you need to kill off a major character or blow up the universe to fix them, you can do it.
It's a terrific article. Be sure to check it out.