Fallout creator got hired at Interplay by knowing D&D rules – and proving it on the spot

Tim Cain is, without exaggeration, a legend among PC RPG fans. His credits include Stonekeep, Fallout, Arcanum, The Temple of Elemental Evil, Pillars of Eternity, and more. And it all started, as he recounted in his latest video, because he out-nerded the competition.

Cain spent a lot of time on tabletop RPGs in his younger years – which probably surprises no one. His love of D&D started early and followed him through college and into a PhD program.

"In the evening, I'd often play tabletop RPGs with grad school friends of mine because we didn't have any money, so we couldn't do anything. So we were kind of stuck in the student housing there. So we played a lot of games, including GURPS, which is where I learned it."

That GURPS knowledge "became important later," Cain explains, because GURPS – the Generic Universal Roleplaying System by Steve Jackson Games – was originally intended as the ruleset for Fallout, before licensing issues pushed the team to create SPECIAL instead. But before any of that happened, a different system turned out to be even more critical to his career.

After leaving grad school, Cain sent his resume to Interplay mostly because the studio was nearby. He ended up getting hired as a contractor, and the reason came down to one interview question:

"Do you know what THAC0 is?"

THAC0 is probably the most famous – and most mocked – rule from old-school D&D. The acronym stands for "To Hit Armor Class 0": the number a character needs to roll on a D20 to hit an enemy with zero armor class, adjusted during combat based on the target's actual AC.

What made the system especially painful was that a lower armor class meant better protection, and negative AC values were common, turning every combat round into an arithmetic exercise. It was dropped in D&D's third edition, though not before leaving its mark on the original Baldur's Gate games.

"They were down to me and another programmer, and we were both about equal in coding skill. But not only could I tell them what THAC0 meant – to hit armor class zero – I then asked them if they wanted the values for THAC0 for each of the four major D&D classes, fighter, magic user, thief, cleric, at level one. They said yes. And I told them the numbers, and I was right. And I got the job."

The irony is that once Cain joined Interplay, he started organizing tabletop game nights – but it was GURPS, not D&D, that took over. The system's flexibility across different settings made it the obvious choice. Cain even built tools to support it, including a character generator and a star system generator.

When Interplay decided to make an RPG based on a licensed ruleset, GURPS was the natural pick.

"I think part of the reason GURPs won is, it wasn't just me pushing forward, it's all those people I played GURPs with at night. And we already had these apps which I pointed out had underlying code that would already jumpstart us into making a GURPs game."

And then came Fallout, and everything that followed.