It was a bug report about a piece of Echo Bazaar content. Polite and well-argued, but blunt. "Thanks," it said, "for deciding my character is a lesbian or something."

As bugs go, that's pretty alarming. What's the worst a bug gets, normally? You tell the player they've been awarded twenty-four moon-pearls, but actually give them two. So they're down a few pretend moon-pearls. Big deal. Or a particular screen looks a bit wonky on their web-browser. So what? Squinting builds character.

RPGs provide a world, adversaries, a supporting cast, and a plot, but they need you to provide one vital detail: the protagonist.

But rewriting someone's sexual preference? That's a problem. And it's exclusively an RPG problem. Infinity Ward doesn't get complaints like that about Call of Duty. No-one ever wrote to Blizzard to ask if it was respecting the life choices of Third Zergling from the Left. RPGs are different. They make the player a creative partner. They provide a world, adversaries, a supporting cast, and a plot, but they need you to provide one little, vital detail: the protagonist .

What makes a game an RPG isn't a character sheet or experience points - plenty of genres use those - it's the sense of ownership you feel over your avatar. Maybe you have to build it from statistics first, give it a class and a name, or just pick from a handful of archetypes. But the process doesn't stop when play starts: Every time you make a decision about who your protagonist is, you're creating. Some players invent elaborate backstories. Most just have a wordless feel for the sort of person their character is or isn't.

For this to work, RPG designers have to leave a hero-shaped hole in the middle of their fiction. We'll call it the identity gap. That's the space that's left to the player; the things they're free to decide are true about their avatar. The contents of the identity gap are a mystery to the game's writers. Anything they write that intrudes on it runs the risk of contradicting details they have allowed - encouraged, even - the player to invent. They can't write about the protagonist's childhood on the grimy streets if the player thought she was brought up in a castle. They can't kidnap the protagonist's husband if they haven't established a marriage. So we come to the impossible question at the heart of every computer roleplaying game: how do you tell a story when you don't know who the protagonist is?

You cheat. Boldly and inventively, like a magician cheats. There are three common tricks.

The first is the Diablo approach: make the personal details of the protagonist irrelevant. No-one cares what your Diablo character's childhood was like because demons are coming out of the cathedral and they are eating your face. In Diablo II it doesn't matter who you are because everything interesting has happened before you get there, and you're just cleaning up. You're always five minutes behind the most epic story ever told. You should've been there. It looked great. If only you hadn't stopped to loot that last Devastating Orb of the Gerbil.