I write, and I also run games.  The writing came first, but only because up until I went to university a combination of geography and lack of social skills meant that I didn’t know anyone else who was interested in RPGs and wanted to do anything more complicated than ‘I hit the orc’ / ‘Okay, you… maybe you hit the orc. I need to look that up on a table’.
There are a lot of posts out there about how running games and writing play off each other – I particularly recommend Penknife’s Things about writing that I have learned from RPGs (part 1) and Part Two. This one’s mine.
Writing and running games use a similar skillset, but it’s not the same skillset by any means. I’m going to talk a bit about how they differ.
Firstly, they both involve worldbuilding, but when you’re running a RPG, you’re allowed to do a lot more borrowing without anyone giving you the fish-eye. You can borrow plotlines from books (preferably books your players haven’t read, unless you’re prepared to put a huge twist in the plot, and if you’re doing all that work you might as well write your own stuff). You can borrow cool scenes from movies. You can borrow characters from anywhere. I borrowed Lois McMaster Bujold’s Sergeant Taura and put her into one of the major Chaos families of an Amber game once.
Whereas, if you’re writing, you mostly have to do your own heavy lifting. If something inspires you… you can use it as inspiration and transform it into something new, or write fanfic. Either is good, and you can do both at once!
If you’re writing genre, there is a sort of pool of shared stuff that you can dip into. I’ve heard it called the myth-kitty, though I can’t seem to Google up who originally said that. If you’re writing SF, you can have some kind of faster-than-light drive without people getting on your case for plagiarising everyone who’s done it before, and if you’re writing fantasy, neither Wagner’s nor Tolkien’s estate will come after you if you include greedy dragons. 
And then, of course, there are grey areas. Sometimes things get into your brain from a book you’ve read and come back out again in your own writing and cause you agonies, and sometimes you think you’ve made something up when you saw it somewhere instead. Sometimes, if you are very post-modern or very sheltered or actively wicked, you think that you can take other people’s clever lines and pin-sharp description and put them in your writing and no one will notice.
But there is an up side to worldbuilding for books, as opposed to worldbuilding for games. You can be much weirder.
I think this is partly because if you’re writing, you have the reader’s full attention – at least until you abuse that privilege – and you can drip your worldbuilding slowly into their ear in exactly the right words, and tinker with the words if they aren’t right. The reader will have assumptions – that if there’s a spaceship on the cover of the book, a spaceship will probably feature in the action and that if the book is marketed as YA there will probably not be a lot of explicit descriptions of sex, that kind of thing – but they’re reasonably willing to strap in for whatever ride you have planned.
Whereas, if you have four players sitting in your sitting-room drinking coffee and eating snack food, those players are there to tell their own story in your world. And that means they don’t just have assumptions, they have assumptions. Some of these are benign, and help uphold the sense of disbelief; your players don’t want to stop the action every few minutes to say ‘Er, does my Roman buy food from a hot food stall, or do his slaves make food for him?’ so they just guess and say ‘Uh, we get a picnic’ and wait for you to fill in the details.
Some of them are kind of neutral: I’ve been running a game for four years in a world where the gnome empire has no percentage at all in funding research into breeding a kind of riding dinosaur that can carry a single human, and much prefers the status quo where there are fast riding-velociraptor things that will carry a gnome, and apatosaur-things to carry humans and freight. My players, who are mostly playing human characters, have just about learned to stop saying ‘We buy horses’ and looking at me hopefully. These days they look at me hopefully and say ‘We buy mounts.’ It’s taken four years to get that far.
And sometimes, the players will assume things that are fatal to your worldbuilding in one way or another, and it is your business to tell them things upfront so they don’t do that. A friend of mine ran a game that was completely derailed by the player party attacking some orcs they met on the road and becoming outlaws, because he’d forgotten to tell them that orcs were peaceable allies of their home nation.
The problem with this is that if people’s heads are already full of ‘How does this new system work?’ or ‘What optional rules are we using?’ or ‘My character is so awesome!’ (or, as the case may be, ‘I haven’t caught up with X in ages! Is he still going out with Y?’ because games are also social occasions) there’s only a limited amount of space for them to remember that in your game goblins have wings and the elves are all raving racial supremacists who ride giant rats into battle.
You don’t generally get this problem with books. Your viewpoint character can have a goblin sidekick who spends all its time fluttering nervously upwards to listen for the ratty scrabbling noise that means the elves are coming, and there’s the background slipped into the action before anyone’s had time to realise what you’re up to.
Secondly, as Penknife says, when you’re writing you can go back and edit. If you suddenly need the Order of the Neither to enter stage left in chapter ten, you can go back and have some of the heroine’s friends mutter darkly about the Order of the Neither in chapter four. Not that this happened to me today, or anything.
If you have particularly dozy or inattentive players, you could try fixing them with a look of injured innocence and saying ‘But you found out about the Order of the Neither when you got that cache of information four months ago!’ But I think this too may count as actively wicked, and also it won’t work if even one player in the group keeps accurate notes.
Being able to edit is a magical thing. I don’t think it’s possible to really appreciate it, until you’re sitting there looking at your notes with half an hour before your players ring the doorbell, thinking ‘My life would be much easier if I’d told them about the Order of the Neither back in August’.
Thirdly, if you’re writing you have to come up with your own protagonists. The most trouble I’ve had with Heavy Ice has been trying to work out which of the various factions on Requite need to be part of the story, and which can be left in the background, and I got round it by staring hard at any characters who presented themselves and thinking ‘Yes, but how much of this story can you carry? Am I sending a background character to do a protagonist’s job?’
When you’re running a game, you need to people the background of the game, and you also get to play mentors and antagonists and love interests and random nosy strangers as needed, but you’re not in charge of the people the story is about. This is really damn hard, and really damn weird, if you come to running games after writing for a while. If you’re planning to run a game in a world you originally created to write in, I’d recommend setting the game a few hundred years earlier or later, so the book’s main characters and the game’s main characters don’t meet: but honestly, I wouldn’t do it at all.
Why wouldn’t I do it? I often get people asking me if I set the games in the worlds I make up for the books, or just assuming I do, but actually I keep them separate. (Reasonably separate. There have been a lot of competent, sarcastic blue-haired background characters, over the years) The reason’s partly what I explained above, and partly that running a game in a world broadens and coarsens it: you get in-jokes and hilarious misunderstandings and by the end of the first evening it’s not wholly yours any more.
Which, don’t get me wrong, is an amazing thing and part of what keeps me roleplaying. Last month one of my players remembered a snippet about halfling cross-labourers that I think I told him in about 2005. This is awesome! But when it starts applying to worlds that I write in, it starts feeling like collaboration, and I don’t do collaborative writing.
And more to the point, the book is for the reader, and the reader probably doesn’t give a damn about the writer’s in-jokes with their friends. Jane Austen could get away with putting an impenetrable joke about Catherine Morland’s father’s name in the first paragraph of Northanger Abbey, but the rest of us aren’t Jane Austen. 
Whoo. That got long. Also I bet that comments, if any, will all be about the footnote concerning knitting. ;)
 When I say games, I mean tabletop RPGs. I don’t do MMORPGs, because there are too many griefers and examples of the phenomenon that Tacky Tramp skewers so brilliantly here, and also because they’re a huge time sink and I already have three huge time sinks in my life: namely, writing, playing Sims 3 and waiting for my husband to finish a sentence.
I don’t mean this as a value judgment: I don’t knit, either, for very similar ‘It’s not that I don’t have the time, it’s that I’m already using the time I’ve got to do something I enjoy more’ reasons. Basically I am really, really selfish when it comes to making time to write in! (And also play Sims 3 and wait for my husband to finish sentences)
 There are fashions in the myth-kitty: blasters don’t seem as popular these days as they were, and everyone who doesn’t read fantasy appears to think that it’s full to the brim with unicorns and drippy princesses when actually it’s a whole lot grittier than it was even ten years ago and you’re likely to encounter one nonhuman race per book, if that.
 You know my feelings about bad Jane Austen continuations, right? They boil down to: it is nice that you love Jane Austen, but if you insist on showing that love by writing terrible imitations of her prose, I am not going to buy your book.
Image © Wizards of the Coast