The best success I’ve ever had in describing RPGs to non-gamers isn’t saying its like fantasy or Star Wars or anything like that. This spiel has actually gotten the light to go on, where they get what it is to play a tabletop RPG. Usually something like this.
So in this game, let’s say you’re a character in a horror movie. You get to make him up, maybe you’re the Blue Collar Plumber who is trying to keep his kid safe. Or the cop that has seen too much. Or the college kid home for the summer to see his folks. But you’re out at the old farmhouse, and there’s a crazed killer with a big machete that’s been stalking you.
I’m running the game, which means I sort of set the scene. You’re at the top of the stairs, and the crazy guy is at the bottom, looking for you. What do you do? You could go and hide, maybe he won’t find you. Or you could make a run for it. Or you could grab a lamp and hit him over the head. Up to you, its your character.
So say you decide you want to make a run for it. You’ll have some scores on your sheet that say how good you are at running, and we roll some dice. If you roll well, you zip past him into the yard. If not, well he gets in your way and now he’s gonna try to cut you in half. If you try something else, we’ll figure out what to roll to see if it works, and then make up what happens next.
The first time I used this approach, the guy who had never played an RPG before had that light go on in his eyes and said “Oh, so the rules determine what happens next in the story! That sounds awesome!”
I find the horror movie approach much more accessible than a specific genre. If you can get them to get it from that approach, then explaining that there are other games that are like that but Lord of the Rings or Star Wars or whatever.