Writing for publication requires both author and audience. Each brings something to the table, and readers have their own sensibilities -- read "axes to grind -- whenever they pick up a book. This is especially true when you work in a shared universe, whether you are doing direct media tie-ins, such as the novelization of a movie, or original stories using franchised characters.
The joy of working on such farms are several: You get to put favorite characters through their paces. Do you know what Darth Vader thinks about while sitting in his hyperbaric chamber, or what Princess Leia looks like wet, just after she stepped out of the shower, or how Conan got his muscles, or where Batman learned martial arts?
How does Ripley feel about androids? How hard is it to plink a Predator? What do Predators think about when they are chasing a couple of future governors -- Ahnahl and Jessie -- around the Amazon rainforest? What countries would Indiana Jones consider moving to because they didn't have any snakes?
What kind of woman turned Sherlock Holmes away from misogyny and got his motor revving?
I know all that stuff. And if you know, it's because I told you.
Plus, the money is not bad, and there is the name-change. Before I wrote Shadows of the Empire, I was "Steve Perry." After that one came out, I was "New York Times Bestselling Author Steve Perry." A dozen times over since, not that it has made me rich ...
I got to read the script for Men in Black months before you saw the movie. I got to have breakfast with Leonard Nimoy, work with the ghost of Isaac Asimov, and some of the animation I wrote is still being shown somewhere in the world every week decades later.
On the other hand, you need look no farther than shared universes to see what axes readers bring with them. Much of my work in such arenas is of the two-and-a-half stars ranking, and usually because they fall into the one-star -- it sucked! or five-star it-was-awesome! categories.
If you find yourself in position to write in somebody's well-known universe -- and let's be upfront about that, chances are that isn't going to happen for most of you -- you need to know going in: No matter what you write. No matter how good it is, nor how happy you and your editors and publishers are with it, you are not going to please all the readers. Not going to happen.
"Princess Leia would never do that!" an outraged reader said to me in a letter. "Why do you say that the Aliens are only as smart as German Shepherd Dogs? You are wrong!" "You had Indy acting like he did in Crystal Skull. I hated Crystal Skull!"
This is part and parcel of the business. Fans build up characters in their minds, and endow them with a reality much more true to them than much of the world in which they live. They know them in ways they don't know their own families. And they get pissed off if you take their characters down a road they don't like. They will argue with you about it, and, in some cases, get further chapped if you don't think that what they think really matters.
Hardcore fanboys know what color the lint in their favorite character's pocket was last Tuesday, and they will argue about it until the cows come home. With writers, and with each other. If you go to a fan website and agree to answer questions about your book, have a look at the stats where it shows how many posts some of the fans have done there.
If you find yourself arguing about stuff in that universe with somebody who has posted fifteen thousand times in the last three years? Save yourself the effort. You can't win. They don't really want to hear what you have to say. They have already built their citadels and armored the walls. You ain't getting through to them.
I'm thrilled to have been involved in adding bits to the fictional lives of revered characters, and happy that most readers seemed to have enjoyed what I did. In the end, it has been worth it, but like most roads, there are the occasional potholes. If you have a chance to travel that road, don't say I didn't warn you.