A 'fic the length of Magnum Opus doesn't spring out of my head fully formed--honest!
Here's some stuff that went into it's creation.
The impetus for writing this came out of TV Tropes, the most addicting website ever, and their "Wild Mass Guessing" page. One of them posited that Team Fortress 2 is a way to get rid of violent criminals. Cue inevitable mental image of the 9 classes in a chain gang (hot!)--even as I was drawing the "What Are You In For" sketch, I was toying with the idea of writing a story around said sketch. I just needed an antagonist--cue GLaDOS, who is also voiced by Ellen McClain (who lends her talents to the Administrator in Team Fortress 2).
(Allow me to wax philosophical for a moment and posit that the true villain of the piece is I, the writer. After all, I'm just as much of a soulless monster for throwing the characters into that situation and doing terrible things to them, and I don't even have a convenient excuse!)
Crab and Raven - the first two characters I came up with, inspired by a sketch of them meeting on the bus ride over. At first, I wanted to name the Spy "Crab" (you may groan), and I also toyed with the idea of "Coyote" and "Wolf" as names, but eventually I settled on "Raven" (for the obsession with shiny objects and trickster god associations) and "Crab" (for the personality).
Crab's characterization, as I mentioned in another posting, is effectively: "James from Silent Hill 2, but with less crazy". I went with a more noble interpretation, though hopefully there are a few hints as to Crab's darker side.
Raven, in turn, is heavily influenced by Arsene Lupin (which was why I toyed with "Wolf" as a name for him). Of all the characters he's probably truly amoral--though of course he can feel fond for people, he has no problems with stabbing them in the back if it suits them. One of the plot points I floated before discarding it was that Raven was a secret double agent for GLaDOS, and he was the one who brought the team together.
The rest of the team were more or less developed in the order of introduction, and as a result not as well thought out in terms of characterization.
The only thing I thought when I came up with Bear was "he should be the most 'innocent' character, comparatively speaking". In my very first TF2 'fic I posited that the Heavy Weapons Guy went AWOL from the People's Army because he had a grandmother who practically raised him, so I nicked that for Bear's personality. Otherwise I didn't really have anything for him in mind besides "he's not ending up with the Medic". (Sorry, Heavy/Medic fans!)
Mouse underwent a complete 180 from the original idea--he started out as a German, and eventually it would have been revealed that he was a Holocaust survivor. Then I remembered that the framing story meant that it would have made Mouse improbably old compared to the others unless he was a kid during WWII, and I was also trying to decide on Box's characterization at the same time. So Box got Mouse's original personality, and Mouse became Hannibal Lecter.
Jack got the "honestly doesn't know what right and wrong is" characterization, which stems from my own interpretation of the Scout class in general. To Jack, the whole mess is nothing more than a great fun game where he gets to kill people all the time and not get in trouble for it, how awesome is that?
Box's particular "quirk" was also nicked from elsewhere. I wanted to write a self insertion story where I was a Pyro (that idea eventually became "Team Fortress and You"), so while coming up with characterizations for both teams I wrote down in a bullet list: "one of the Engineers' fatherly and warm air has creepy pedo vibes in it that may or may not be genuine". For Box I went the whole hog--GLaDOS' rant about how none of the team should ever be released is as much my opinion as it is hers, and I wanted the readers to be conflicted about rooting for the characters to escape GLaDOS' grasp.
Poor William Wallace has no real characterization outside of "angry black man". I hereby apologize to all black Scottish cyclops everywhere and vow that someday I'll be able to write Demoman without relying on character archetypes. This 'fic...was not that day, unfortunately.
Etna, didn't get a lot of words dedicated to what was going on in her head, as most of the story still ended up rather Crab- and Raven-centric. If I do get around to fleshing her out some more I'll probably have her switch moods at the drop of a hat, alternating between bitchy and flirtatious sometimes in the space of a single sentence. (In other words, the classic anim* "Tsun-Dere" personality, except with more stuff getting set on fire.)
Last but not least, I wanted to give Sarge a reason for his mental instability other than "he is batshit insane". This probably makes him the most outside canon as far as characterization goes. He didn't get a lot of screen time, either, because I couldn't think of any good lines for him.
2: Plot points
The original planned ending was a lot darker--the team would try a few escape attempts, get foiled, and then decide to pull a Jonestown-style mass suicide as a final "Fuck You" to GLaDOS.
(Back when Mouse was still German, I had plotted out a scene where he suffers a nervous breakdown and reveal that he has epic levels of survivor's guilt due to his family ratting out other Jews during the War, but then he ended up going to a concentration camp anyway and both of his parents died. None of that made it into the final story, obviously, but I like this idea enough that I might use it somewhere else.)
But GLaDOS was rapidly becoming borderline omnipotent, and it didn't seem likely that the team would be able to kill themselves in any manner that didn't involve them being respawned soon afterward. It also seemed a bit out of character that they'd all agree to a suicide pact, and at this time I was reading Cat Bountry's take on I Have No Mouth and I Must Scream, so I started shying away from the "Kill Them All" ending. At the last minute I decided on a more ambiguous fate for the main characters, leaving it up to the reader to make the call.
I also ended up skimping on a lot of details with regard to relationship dynamics. Right now I don't really want to work on this story any more, but depending on the feedback I get I may try to expand on the plot a bit more so that the porny bits I hope to write will make a bit more sense.