Warning! Portal 2 spoilers below.

Valve's Chet Faliszek and Eric Wolpaw have revealed many paths that Portal 2 didn't take at a very entertaining post-mortem at the Game Developers Conference in San Francisco.

Wheatley - the chatty, idiotic AI personality voiced by Stephen Merchant, who eventually becomes Portal 2's main villain - originally stayed dead after GLaDOS kills him at the end of the first chapter. The player would then have gone on to meet six other spheres.

The pair also revealed that the ending of the game was initially radically different, that it was going to have multiple false endings, that the co-op campaign originally had a much more involved story involving a parody of comic strip Garfield and that Valve briefly toyed with a terrible competitive multiplayer mode.

"Originally GLaDOS crushed Wheatley right after she wakes up and that was it, you didn't see Wheatley any more. He was just dead and gone." said Faliszek. "This was back when GLaDOS was still going to be the main antagonist. After Wheatley died, you were going to meet six other spheres, each with their own personality."

"As an example, this was one of them, something called the Morgan Freeman sphere," said Wolpaw, showing a still of a sphere sitting on a pedestal in an empty room. "So he'd been sitting on that little pedestal for a few centuries, and he was just incredibly, incredibly wise - but only about the 20 by 20 space that he was in.

"So as soon as you dragged him 22 feet out of the room, his mind was blown and he was pretty much useless. Although as the game progressed, he eventually got his feet under him and started delivering some homespun wisdom that all related back to this 20 by 20 space."

At another juncture, the audience was shown a paranoid sphere who had fortified a whole area of the Aperture complex - but all his defences pointed in one direction and the player could easily drop in behind him and pick him up.

However, Faliszek explained that people testing the game missed Wheatley and none of the other spheres had time to bond with the player. "We were missing something - even in a talky game like Portal, you're still more or less writing in the margins. Our simple answer was, let's just kill Wheatley, and bring him back." In fact, it was this decision that eventually led to Wheatley becoming the antagonist for the latter portion of the game.

The co-op campaign was originally set after the events of the single-player game. "We actually thought we had a pretty good story going," said Wolpaw. "So with Chell gone, GLaDOS is just prepared to test for eternity with two co-op bots that we introduced in single-player.

"Unfortunately, she quickly discovers that none of the tests she does is providing new data, because without any human observer, the tests are always in this sort of Schrodinger's cat state of quantum uncertainty.

"So what she does is she starts sending the bots into the bowels of Aperture to collect human artefacts in the hope that they will help the bots become more human so she can break through this kind of quantum problem she has."

The first artefact retrieved was a cartoon spoofing Garfield, called Dorfeldt. "The bots find it, they bring it back to GLaDOS, and none of the three of them can figure out what's funny about this Dorfeldt," said Wolpaw. "So GLaDOS rewrites it to make it funnier.

"In the first three panels, Dorfeldt is sitting next to an empty can of lasagne. His owner comes and is really angry that Dorfeldt ate the lasagne. And then the owner says that he's activated the neurotoxins in the room and Dorfeldt will die." The next two panels are Dorfeldt thinking, 'I've made terrible choices in my life,' before keeling over dead.

But, Falizek explained, Valve discovered fundamental differences in how players relate to a story in single-player and co-op. A solo player is "a captive audience - they will watch intently and pay genuine attention to the crafted experience. Co-op players, on the other hand, will interrupt your best material to ask each other what they had for dinner." This meant they had to radically simplify the story and repeat the main points in little bits (an interesting contrast with the puzzles, which the designers found they could make more difficult for two players).

"We also tried a competitive multiplayer mode which we put together over the space of a month or two," Faliszek revealed. "It was kind of a mix of the old Amiga game Speedball and Portal, except with none of the good parts of either of those two. The game was super chaotic and no fun, so the only good news about this part was that we cut it pretty quickly."

The writers' final challenge was to find an ending to the story. They experimented with one in which Chell would have to finally speak the single word "yes" (by pressing the middle mouse button) to deactivate Wheatley, but although it sounded funny on paper, "boy did it suck", said Wolpaw.

The eventual moon ending was actually one of a series of false endings that they had planned to include in the game, similar to the fire pit in Portal 1. "A small percentage of playtesters were just fine with riding into the fire pit, that was a good ending for them. It was dark, but they liked it, so we thought, we'll service those people," Wolpaw said.

"So we had these parts throughout the game where Chell would die and that would be the end and we'd play a song, and if you wanted to you could just quit there. We had one that was like two minutes into the game, and if you died there, there was a song that was just about reviewing those first two minutes."

Later, there was a part where you could see the moon and portal to it, "and you would asphyxiate while listening to a sad song about the moon." They eventually cut the alternate endings because they would be a lot of work and felt they didn't have enough good ideas, but the moon idea was repurposed as the true ending of the game. "It was this perfect mix of being totally awesome and completely stupid," said Wolpaw.

"One of the biggest lessons we learned is try to give yourself enough time for the obvious to become obvious, because at a certain point, most of the answers to what you need to do are buried in what you've already done.

"So that was the last major puzzle," concluded Wolpaw. "We finished the game, we released it, and this year there was a pretty clear winner of most game of the year awards.

"And that was Skyrim."