What is more fun and rewarding for you, writing your own characters
or writing characters you've been given from someone else?
Ooh. That's a great question. Actually it's two great questions -- "Fun?" and "Rewarding?" (since they might not be the same thing) -- but luckily in this case, the answers are the same for each of them.
In my circle of writing friends, accomplices, and employers, I've learned to call writing characters that belong to someone else "playing in someone else's sandbox."
Let's look at both...
Writing my own characters is always fun because all the ideas are mine to waste, use, destroy, or run with. There's a freedom in that playing in my own sandbox gives me that playing outside it just doesn't or can't. I imagine me writing Dominatrix for Gene Simmons (and IDW, of course) must be a lot like Gail Simone or Palmiotti and Gray writing for DC's Batgirl or Jonah Hex respectively. There are limits to what you can do with the characters, and Mom and Dad have to approve it, whereas me writing Fishnet Angel gave me limitless stories to explore. She was completely at my mercy and didn't have a copyright holder to protect her from my machinations.
My own characters are also rewarding to write because I get to see people react to new characters and stories they've never heard of and never seen. And I get to explore themes and ideas that are often personal to me through them.
However, I also have fun playing in someone else's sandbox because I often am allowed to bring a slightly different viewpoint into play that allows even the person or company who owns the character to see him or her in a new way. This can be especially fun when it walks that trickly line between staying the same and showing too much new too fast. It's an art, not science, that is very rewarding when you get it right, and can suck eggs when it overstep your bounds. A case in point, Martheus Wade is giving me a pretty fair bit of latitude to write his character Turra: Gun Angel. He has had to rein me back in a few times, though, and learning how to work within the reins is all part of the give and take that comes with playing in a sandbox not my own.
What can be most rewarding about writing others' characters though is this -- there's no feeling quite like knowing you're being trusted with someone else's property, to change it, grow it, and leave your own mark on it. Whether it's Marvel trusting Brubaker to control the fate of Captain America for a while or Pulp Obscura trusting me to tale brand new tales of Armless O'Neil (coming soon!), it's immensely gratifying to have that trust from from a publisher and sometimes from a friend.
But, to answer the question completely, I have to say that the most fun and rewarding is when I am being trusted enough with someone else's toys to bring a new toy of my own creation into the mix and have it become part of that sandbox. Case in point, when I created the Saint-Devil, Monique San Diablo for my novella in Lance Star: Sky Ranger Volume 3, I asked Bobby Nash, who owns Lance as a character about my new femme fatale for Lance, and when he read the story, he liked her enough to allow her to become part and parcel of Lance's mythos, to the point that in my new story for Volume 4, she's back, and will also have another adventure with Lance in a comic book short in the upcoming second issue of All-Star Pulp Comics!
And even better, reader response has been exciting.
So, for me, the most fun and rewarding option is to combine the two. (Come on. You knew it wouldn't be a simple one or the other kind of answer for me. Right?)