In most co-operative games, players don't work together so much as work beside one another. The closest you'll get to real teamwork is pulling the trigger at the same time. Portal 2 doesn't work that way. Its co-op problems are impossible without a friend, and each reality-twisting solution forces you to share a brain.
My brain is neurotic, and though he hopefully never noticed, playing with Tom was competitive, too. Every time he worked out the solution first, it stung. Every time my suggested solution turned out to be wrong, I was convinced he thought I was an idiot. The problem is that you're never just wrong in Portal 2, your idea is stupid, deadly and physically impossible.
Thank god it's also funny. In singleplayer, Portal 2 is a finely scripted sitcom starring a woman, a robot and a potato. In co-op, it's a slapstick buddy comedy, with both players as comic foil and GLaDOS as your straight man. When either of us would screw up, Tom and I wouldn't yell or criticise one another – we left that to Owen and Tim, who were playing at the same time. Instead, we'd laugh, sometimes make P-Body and Atlas high-five, and leave my brain to find reasons to be paranoid on its own.
Having a friend along cancels out all the loneliness you feel in Portal 2's singleplayer. It's a deliberate part of that experience that Chell is isolated amidst the world of test chambers, but it's not always a relaxing way to spend a few hours in the way the co-operative mode can be. Once you've completed both, you're also far more likely to return to the co-op mode a second time than you are the singleplayer. Even knowing the solutions while playing with someone who is on their first run through is fun, as you get to step back and play shepherd to someone else's enjoyment.
As much as acting out the solution is kinetic and wonderful, it was the thought process I enjoyed most. Tom and I would walk in to a new challenge and think: “Um, wait, how do we do this?” We'd both stand still, playing the level through in our mind, once, twice, wait, I've got it! If I place a portal here – foont! – and then another here – pshoon! – then I can cover that floor with slime. Then, if I place two new portals at either end – foont! pshoon! – and now you run between them. Woosh. Woosh, woosh, wooshwooshwooshwoosh. And now I place the exit portal here – pshoon! – which will – Wheeeeeeee!
Read our Portal 2 review for more.