Here's how Wikipedia defines fan fiction, for those who may be unfamiliar with the term:
Fan fiction (alternately referred to as fanfiction, fanfic, FF or fic) is a broadly defined term for stories about characters or settings written by fans of the original work, rather than by the original creator. The term usually applies to works that are not commissioned and unauthorized by the owner/creators and publishers of the original and always works of which are not professionally published. Fan fiction is defined outside of original fiction, which exists within its own discrete, professionally published universe, and therefore, as long as it's related to the subject, can be completely outside of canon works within that universe. Most fan fiction writers assume that other fans are reading their work, so their readers have knowledge of the canon universe (created by a professional writer) in which their works are based.
I've been out of the closet as an ex-fanfic writer for a few months now, but when I first started submitting my work for professional publication, I kept it under wraps. Some editors don't care if you've written fanfic, whereas others frown upon it, considering it the mark of a dilettante. Even after I sold my first novel, I kept it on the down-low. I had this awful fear that if my publisher found out, they'd scream, "Poseur! We thought you were a real writer!" and promptly cancel my contract.
What I didn't know at the time was that a number of published authors started out writing fanfic. Romance author Susan Krinard made a name for herself as a writer and an artist in Beauty in the Beast fandom back in the late 1980's. (In fact, when I met her at the huge charity book signing at RWA last August, I told her I still had one of her gorgeous Vincent and Catherine t-shirts. It doesn't fit me anymore, but I still have it!) Susan Sizemore used to write Forever Knight fic. One of the biggest m/m historical novels published in 2008 was based on that author's Pirates of the Caribbean fan fiction.
The upshot is, I shouldn't have been worried - fanfic authors turning pro are apparently legion. In the interests of full disclosure, I'll clue you in on what probably isn't much of a secret if you've read my first novel, The Arrangement - it started out as fanfic too. If you're familiar with the TV show Smallville, it's fairly obvious that my two heroes, billionaire CEO-turned-US-senator Eric Courtland and his lover, newspaper reporter Nick Thompson, were initially based on the characters of Lex Luthor and Clark Kent. But by the time I got done rewriting (and rewriting and rewriting…), Eric and Nick had taken on a life of their own, independent of their fanfic origins - and ideas for four more books in the "Courtland Chronicles" universe sprang into my brain!
Getting that first book published, however, made me realize I still had a lot to learn about making the transition from amateur to pro. You can deal with edgy, even off-the-hook subject matter in fan fiction, but that doesn't mean a mainstream audience will embrace it. The list of verboten topics includes: incest, bestiality, underage sex (no publisher I know of will allow a sexual relationship involving a character under eighteen) rape and other non-consensual sexual situations, pedophilia, necrophilia, scat, golden showers (and don't even ask about the inexplicable kink for male pregnancy). Fanfic is rife with this stuff, but it doesn't fly in professional fiction.
(To be clear, my novel doesn't feature any of the, uh… objectionable material cataloged above. And with the exception of an old X-Files novel, which featured a fairly graphic rape scene, my fanfic was relatively tame. Sorry to disappoint! However, my m/m novella, Strictly Business, contains some BDSM activity that's consensual, though not entirely safe or sane.)
Another thing I had to learn through several rewrites was that my style of writing tended to skimp a bit on physical descriptions of the characters and settings. This is something fanfic writers tend to gloss over, since we're dealing with ready-made universes where it's understood that everyone already knows that, say, Clark Kent is a tall, dark-haired hottie with superpowers who lives on a dairy farm in the middle of Kansas. In fanfic, we don't need to spell these things out - but in professional fiction, we do. It seems like a pretty simple thing to grasp, but it took me awhile - and I still don't particularly like writing description. My hat's off to writers who can work it in and make it look seamless.
But the biggest thing I had to learn about turning pro had nothing to do with the actual process of writing. That part's easy for me. I'm in my element sitting on the couch with my laptop, tapping away on the keys. It's putting myself out there to promote my books that I find difficult. A few months ago, when I had to do my first online chat… well, let's just say I would've preferred having my fingernails yanked out with rusty pliers. I've never quite grasped the art of schmoozing. Most writers say they dislike the solitary aspect of their jobs, but that's exactly what I do like about it. I've never been a people person. The thought of jumping up and down (metaphorically, of course) screeching, "Hey, look at me! I've got a book out! Wanna buy it?" makes me want to crawl in bed and pull the covers over my head.
That's why it took me seven months after the publication of my first book to screw up my courage to start a pro writing blog. I've had a private one over on LiveJournal since 2004, but it's filtered down to a small circle of friends, and that's the way it's staying. Honestly, I'm not sure whether this grand experiment with letting it all hang out in public's going to work, but I'll give it a try for a couple of months.
Hopefully it will help boost sales of my books, if I don't end up boring people to death. ;)