This topic has been done to death and beyond, but in creating yet another blog about RPGs, it's fitting to put my own feelings on the topic in writing, if only for posterity's sake.
My opinion on the definition of a "role-playing game" is hardly unique -- really, just interactive storytelling -- but over the years I've thought less about what an RPG is and more about the best way to bring new players to the hobby. Defining RPGs is easy. I've found through experience (unfortunately) that proper introduction is much harder. It's a very complicated activity with a fair amount of time investment and delayed gratification.
When first explaining the idea to the uninitiated, I like to start by focusing on the concept of a story. Imagine, for example, a cliche horror story told among friends huddled around a campfire. Obviously the experience benefits from a good story, more so from a good storyteller. However, the quality of the story isn't the point; it's primarily a social activity -- a bunch of friends spending time together. As far as priorities go, telling the best possible story is a distant second to everyone having fun. This is why I believe how the story is told matters more than the story itself.
RPGs take this one step further, and that step is interaction. The storyteller still dictates the flow of events and paints the scene, but now everyone else is each allowed to control the actions, choices and dialogue of one character in the story. (The storyteller -- i.e., "gamemaster", or GM -- controls all other characters.) Don't want the naive girl to go down the dark staircase alone and unarmed? Fine, she doesn't -- unless the participant ("player") says otherwise.* There's nothing stopping the player from deciding the naive girl has had enough and spends the rest of the evening in a fortified bunker (assuming she has access to one). However, it's important to keep in mind that giving players control over characters doesn't remove each person's obligation to keep it fun for others. If anything, "role-playing game" is a misnomer because there is no "game" to win; this is a story. Once the GM establishes the tone, players should not make character choices purely for personal amusement. How the girl winds up (alive or in pieces) doesn't matter as much as everyone having fun, and it's not fun if she spends the rest of her days in a bunker. That should not be confused with the expectation that the girl should go down the dark staircase anyway. At the very least, even though the character can deal with the monster in the basement however the player sees fit, the player should remain engaged.
That, of course, depends on the GM giving the players quality material to work with. Otherwise the players are just as trapped as actors dealing with a bad script. If avoiding the basement is the obvious choice, then there really isn't a decision to make. Generally, if the setting is devoid of any objects, characters or events for the players to interact with short of throwing reason out the window, there's no story. Beyond that, though, the players should make an effort to invest their emotions in the setting created by the gamemaster. It's natural to care about the character one controls (though some don't), but I have observed many players treat the gamemaster's ("non-player") characters as expendable. This has the effect of weaving a story about a group of psychopaths. That's the players' perogative, but the cost is a lack of immersion. If no one cares about anyone else, a risk-averse strategy follows, resulting in a lack of dramatic tension. To demonstrate this, let's go back to the cliche girl-and-the-basement example. Let's add a plot hook -- the girl's boyfriend went first to investigate, but fell through a rotted step and now lays injured at the bottom of the stairs. A sufficient emotional connection to the situation naturally leads to the girl overcoming fear (and possibly reason) to rescue her injured boyfriend. It's cliche, but at least something is happening. However, in RPGs, it's more likely the player will say "fuck it" in a fit of sociopathic self-preservation and leave the boyfriend to fend for himself. It may be a rational decision, but it's not necessarily a human one. Self-preservation is fine as a character motive, but exciting stories invariably involve taking risks. Who tells a campfire story about a girl investing in municipal bonds? The story is more important than keeping the characters alive at all costs.
So, say the girl does go down the stairs, and encounters the monster in the basement. How does this resolve? It's easy for the player to say the girl is carrying a shotgun and shoots the monster in the face, but where's the tension in that? This is where there rules of the "game" finally matter. They impose order for participants disagreeing over the details, and in doing so they add an element of risk. Does the girl really have a shotgun? The player is required to keep a list of her possessions in writing. If it's not on the list, she doesn't have it. Even if she does, who's to say she'll hit her target? How skilled is she at handling firearms? Assuming the shot hit, how much damage did it do? What sort of damage can the monster take? There are rules for all of these questions, and in letting the rules (and dice) dictate the outcome, the fate of the girl is taken out of everyone's hands and ultimately left to chance. The most anyone can do -- GM or player -- is tip the odds for or against her favor. Everyone holds their breath as the dice roll, and as the excitement and suspense build, the previously uninitiated now understand why gamers choose to spend hours surrounding a table covered with papers, rulebooks and dice: It's a matter of life or death.
*This is where my explanation of RPGs stopped, until recently. However, lately I have found that I missed an important detail in emphasizing the importance of a collective effort.