Have you played Walking Dead yet? You must. But you must also brace yourself. I’ve just played episode three of the developer’s staunchly faithful comic book adaptation and it is by far the most arresting, heart-wrenching video game experience I have had in a number of years.

The plot follows escaped convict Lee Everett as he joins up with a small group of bickering survivors amid a zombie apocalypse. Early on in the story he discovers a little girl named Clementine who he decides to protect, and from here his life is a series of nightmarish encounters with undead monsters and the uninfected who have essentially become monsters of a different sort in their savage determination to survive.

While there are simple puzzle elements and a few exploration sequences, this is really a game about decisions. As in a lot of action role-playing games, each conversation provides you with a multiple-choice response list, allowing you to react to other characters with kindness, anger or indifference. But… BUT… there are also these horrific set-piece sequences where two of your group are attacked at once and you have to make a choice – with a timer ticking down – about which to save. Then you get to see the other one being eaten to death – you know, just so you completely understand about finality.

The genius of Walking Dead is that its decision system does not work like similar systems in other games; there are no right or wrong choices, there are no good or evil ones. There are just stark instinctive dichotomies. Certainly, in Walking Dead, you do what you think is ‘right’ but then instead of judging you, the game shrugs and changes the story accordingly. Just like your characters, you are operating in a moral vortex to which civilised values cannot be ascribed. Life is survival for these people and when it comes down to it, survival over-rules morality.

There is a branch of economics known as behavioural decision theory, which tries to factor in the subjective nature of human choice to formalise the logic of the decision making process. Here, the maths would be wildly complex. Sometimes I make my decisions based on age (save the kid, the kids need to be protected), sometimes based on medical altruism (save the wounded guy, the others might be able to run to safety), sometimes I make rash decisions based on fallacy and stupidity (save the girl, she’s quite attractive). Freed from the tyranny of an overtly moral system, I sway about the place like a confused bully, smiting my fellow survivors for perceived slights or possible weaknesses, or going out of my way to do something merciful at the potential risk of death.

Choice, I have discovered, is terrifying. It’s not the zombies, it’s not even the cannibals. It’s the knowledge that at any moment in this game, I am going to have to do or say something culminative and potentially catastrophic. I know it won’t be game over, but I will have shoved the story down a branch that defines me as a gamer and a character, every thing I do is judged by the person who’s in the worst possible position to do it: me.

And there’s an extra layer. In most games involving story-splitting choices, you’re thinking ‘okay, what do the game designers want me to do?’. We’re very rarely trusted to actually decide on stuff that isn’t a digital proposition, that isn’t right way/wrong way. But we’re still conditioned to think in terms of entertainment systems so when confronted with a highly narrativised adventure that doesn’t judge, we revert to linear forms; I think we become cinematic.

In Heavy Rain for example, I became obsessed with the limited scope of each scene and the mannered, subjective nature of the camera work. I would often self-consciously ‘act’ while making decisions for characters, wondering around a room, leaning on a desk, then wandering off again – I felt like an actor and behaved as such.

But in Walking Dead, it’s slightly different. Now, when I fumble for some sort of logical basis for decisions, I think, ‘what would Lee do?’ I have constructed this fictional personality in my own head, and I often do things in the game that comply with this image. In short, I’ve created my own ruleset and it is about maintaining the veracity of the character. I am writing the story as I go along.

And that is weird, it’s scary. In Walking Dead we have a game that is at once highly structured and formulaic (it is really a hoary old point-and-click adventure) but also nihilistic and chaotic – we have a game in which the structure communicates the narrative as much as the plot does. And ultimately, we have a game that understands something intrinsic to human decisions – often, we don’t do things because we think they’re right, or even because they’re wrong; we do them based on unconscious rule sets, on prejudices and fears, on unspoken needs and on bargains with fate.

Choice is terrifying. We spend a lot of our lives unconsciously trying to narrow down our possibilities. And a game that tells us that survival is full of choices, and that they won’t be pleasant, and that no one will pat us on the back and say ‘well done, you saved the president’… that is fear incarnate.

Horror is not about knowing you have to be strong and brave to make the right decision; horror is about finally understanding that there is no such thing.