Here is how it goes: I have these two friends, and we all share a brain together. We are all writerly-types - though some of us a artistly-types as well, because we are a gifted bunch - with similiar tastes in consumable media. My friends and I tell each other stories. Usually this involves sitting around and just randomly ad-libbing bits of dialogue at one another in funny accents, or coming up with wacky scenarios. Sometimes it involves costumes, props and a video camera. You musn't judge us.
This particular one did involve costumes, actually. There's a whole steampunk subculture we were starting to get into about a year ago, and we came up with characters - three women running a sort of ad-hoc detective agency in an alternative Victorian England. We were the characters, to a certain extent, or at least we each took the role of one of the character primarily during our little story-telling sessions. I don't think it ever occured to any of us to put male characters in - the female costumes were so much cooler, and the characters more interesting. (I find that female characters in a culture that is extremely patriarchal are more interesting, because they have more of a struggle and conflict, but that's just me - the male point of view still tends to dominate so much of our culture)
ANYway. So we spun this story-situation of these women having wacky adventures and building things and wearing excellent clothes and drinking tea between blowing things up, but never developed it beyond a kind of abstract idea we might like to do for Halloweeen. And then we abandoned it when the next shiny thing caught our imaginations - I think it was a radio play set on the most boring spaceship imaginable, with Welsh accents. Which requires far less props than steampunk, and also allowed us to shout "Cardiff or bust!" in a completely non-ironic manner. This was around January-ish, I guess.
All of the writing itself was mine - we never wrote anything down in these storytelling sessions, or at least I didn't, though we would occasionally pull out a microphone and record ourselves (don't judge us), so though I had the ideas and the characters solid in my head, I had to write the narrative from scratch, including most of the dialogue, speech patterns, and the weird humour that seems to creep into most of my work.
A collaborative tool I use a lot, even when I'm not working on the twisted three-way brainchild of me and my besties, is an online word-processer which allows several different people to remotely access a given document. I use googledocs, emailed the link to my braintwins, and watched as they added giant purple text to the document in the form of comments like "HOLY SHIT I FORGOT ABOUT THIS HAHAHAHA" and "needs more explosions" and "shouldn't we have a butler? I want a butler. We can call him Sneeds."
Our current project is a comic about the three of us living in an apartment building full of geeks. LIke most of our collaborations, it is only funny to us.