A common trait of Western-style RPGs is the ability to choose what kind of person your protagonist is through their actions. In games like Dragon Age Origins, the Knights of the Republic series, and the Mass Effect series, you have the opportunity in most of the game's situations to choose how your character will respond to dialogue or deal with a situation. This typically boils down to your character being a super nice, understanding fellow who saves everyone always, makes everyone feel good about themselves, and walks on water, which he has turned into wine...or a complete and total douchebag who destroys everything, pees on puppies, and eats children.
It's a fairly good idea, making the games into actual Role-Playing Games by giving you some control over who your character is, and the RPGs I've played usually do a good job of it, allowing for quite a lot of distinction between the characters you can create.* But Fallout 1 and 2 gave players a 3rd option--you could be Good, Evil...and Stupid.
Basically, when you start a Fallout game, you get to choose the stat build of your character, deciding what areas he/she will be strong in, what his/her skills will be, etc. This, of course, affects the game in many ways, and a character with higher Intelligence will have more and better dialogue options in many situations. However, the Fallout designers had some fun with this idea, and made it so that having an extremely low Intelligence score (3 or below) would actually get rid of most normal dialogue options, and replace them with entirely new ones--the often meaningless babble of a complete imbecile.
And man, is it hilarious.
Although you only get a few really funny moments in Fallout 1, Fallout 2 is just full of extremely amusing situations arising from a world-saving protagonist who is dumb as a stump. Stumbling around the post-apocalyptic wasteland as a simpleton more interested in ice cream and shiny objects than with their mission and the fate of the world is just loads of fun. Granted, you don't get to do as much stuff, as many people don't want to entrust all their problems to a grinning moron, but there're still plenty of side quests available to a stupid character, and all the necessary stuff on the path to completing the game will work out for you. It's actually funny to watch a stupid character basically manage on dumb luck to do everything a regular character has to work at to save the world.
There are even occasions where being a stupid character makes parts of the game easier, or provides a better reward. A stupid character can more or less just walk into Vault City and gain access to its Vault, where normally you have to buy or fast-talk your way into the city, and then do a long quest or have crazily good stats to get into the city's Vault. A stupid character will, in the town of Modoc, get paid with a partially eaten cookie for a pest control job, an option not open to a regular character. You wouldn't THINK that's actually a good thing, but the cookie is a very rare item that can temporarily boost how many actions you can take in combat, which is a pretty big deal in a tough fight. And in San Francisco, a stupid character who completes the Brotherhood of Steel's tasks will be rewarded as a regular character would, but be given the bonus of having the tanker ship fully fueled, something that a normal character has to do an extra quest for.
So the game does play a bit differently here and there for this third character path. More importantly, though, the dialogue is just absurdly funny quite often. In a generally dark and serious game, you get to watch a nitwit run around and...
Have his/her feelings hurt by jeering 10-year-olds, and try to get them back by telling them that he/she is going to go to a party with cake and ice cream and presents and that THEY aren't invited.
Walk off in the middle of an involved conversation because he/she becomes distracted by some nearby sand.
Obtain plot-essential computer parts only because he/she is hungry and they look like electronic Pop-Tarts.
Get electrocuted while exploring the insides of a computer, having confused an automated voice for a woman trapped inside the computer in need of rescue.
And so on. It's a really fun third alternative to the usual Good and Evil way of playing through the game, giving the game not only a lot more replay value, but also an entertaining extra perspective that helps to emphasize the games' tongue-in-cheek humor, which is almost as large and important a component to their storytelling as the serious and dark aspects. I was really disappointed that Fallout 3 eliminated the option for a stupid character, but at least the upcoming Fallout: New Vegas is supposedly bringing it back--although I'm not sure, from reading about it in Game Informer, whether or not it will be in a significant capacity. Still, here's hoping.
* Though not all of them--Risen's protagonist, while not lacking a personality, is fairly mild in general and doesn't seem to vary too much in how he acts regardless of what you choose his actions to be.