Even more mind-blowing, if I met a second guard and got him to follow me as well, my two companions would have conversations. Sure, they were stilted and basic, but having two video game characters chat amongst themselves, completely ignoring you, was amazing.

That, I think, is the core of what made Half-Life such a special experience in 1998: the world gave the impression of existing apart from you. Instead of a game where everything exists just to interact with you, there was a sense that the Black Mesa research facility was real, and huge, and people actually worked there.

This sense of a dynamic world that would carry on whether you were around or not was further aided by the soldiers and aliens in the game doing battle amongst themselves. I had never played a game in which I could simply keep my head down and let two independent enemy factions duke it out.

Half-Life also featured astonishing artificial intelligence, especially in the hostile marines. In 1998, the standard video game enemy would run toward you idiotically, heedless of danger. In contrast, the soldiers invading Black Mesa would work together, flush you out of cover with strategically thrown grenades, take cover while reloading, and even shout out messages to each other. The net effect was that you were being intelligently hunted, a simultaneously thrilling and unnerving experience.

The overall effect of all of this, though, was the story. Despite featuring a totally mute protagonist and no voice-overs or on-screen narration, Half-Life was carried along by a strong plot. Snippets of story would give you context as you played through - a helpful scientist would give you directions, or an abandoned military radio would blast out instructions to the marines.