Choose Your Own Adventure
The ability of players to affect events in a game is one of the crucial components that makes an RPG what it is. It's also one of the aspects of GMing that's easiest to mess up. I'll talk about why player choice is so important in RPGs and offer a few suggestions for how to uphold it in your games.
Of the many topics of conversation that recur among gamers, tales of bad GMs of games gone by is one of the most popular. Most of us have experienced at least one of the classics: the killer D&D DM who seemed to delight in throwing PCs up against unkillable monsters and unsolvable puzzles, the GURPS GM whose lack of creativity turned the game into to a death march of random encounters, the World of Darkness ST so enamored with the doings of his badass NPCs that the PCs never got the chance to shine. While all bad GMs are unique in their own frustrating ways, what many people don't realize is that most of them have one crucial thing in common: They don't allow players to make meaningful choices with the potential to change the course of the game.
Choice is a good thing in all gaming circumstances.
Here's the key: Choice is a good thing in all gaming circumstances. The more of it you have, the better. The instant the players can no longer affect the story in any meaningful way, your group isn't playing an RPG anymore; they're listening to the GM tell a story around a campfire. That can be enjoyable, but it's not why we sit around a gaming table. Instead, we sit down because we want to help tell stories where our characters are central to the action, and to control the course that story takes through decisions we make. The decisions we find important may vary with the style of gaming we prefer - Which feat do I take to optimize my fighter's build? Do I side with the king or the rebels in this political conflict? Does my character marry the prince and settle down, or keep adventuring? What equipment should we carry to ensure that our party survives its trip through the wilderness? - but they're always our decisions. And the more real those decisions are - that is to say, when picking one side of a conflict, or stocking your pack with certain items, or choosing particular feats has a real impact on the outcome of a story that would not have occurred otherwise, as well as the potential for disaster - the more engaging and fun a game becomes.
Upholding player choice is antithetical to the doings of bad GMs everywhere. In the previous examples, it's easy to see how the WoD ST's style doesn't afford the players a choice; when game sessions consist of sitting around and watching powerful NPCs do all the heavy lifting, the GM may have fun, but the players will be bored out of their minds due to their inability to affect the game world in any meaningful way. But it's important to recognize that the other two styles of bad GMing mentioned are also the result of an essential lack of focus on player choices. The killer DM sees gaming as a "me vs. them" competition rather than a collaborative pastime, and builds his adventures firmly within that idiom without regard for his players' desire to succeed every now and then. The uncreative GM has (perhaps inadvertently) limited the PCs' experience by failing to give them an interesting setting with which to engage. Despite their different game-running styles, bad GMs are, at a basic level, afraid of what might happen if they gave up any control to the players in their game. The result is unenjoyable for anyone who sits down at the table.
Much of the time, failing to give the players enough of a choice in how they carry out their own fun is the result of oversight rather than malice. But the worst GMs and systems are the ones who outright block players when they make choices the GM doesn't agree with. This is most clearly seen in the form of those single-minded GMs who seem to get a kick out of frustrating players' desires. A friend of mine once played in a D&D game in which the players decided to sink a large amount of time, gold, and XP into creating magic plate armor for the entire party. They spent the better part of a session playing through the item creation process, with the DM aware of and approving every step in the process. In the very next session, as soon as the PCs left their mountaintop lair they were immediately ambushed by a group of rust monsters. The magic armor was completely destroyed. It should come as no surprise to you to learn that the players abandoned this campaign shortly thereafter - all because the DM decided that he couldn't or wouldn't respect a choice made by his players about how they wanted their characters.
Systems-wise, a frequent manifestation of disdain for player choice can be seen in broken critical failure mechanisms. Think about it: when a player puts a large amount of character creation points or XP into a particular skill or power, they are sending a message to the GM that they have chosen to play a character who excels in that area, usually at the expense of other skills. It becomes incredibly frustrating for random chance to make a character who is supposed to be very good at one thing look worthless fool because of nothing more than unlucky die rolls, and thereby undermine a player's choices by turning their expert archer into a giant buffoon. This is especially true of systems like d20 where the mechanics state that no matter how good your character is at something, 1 out of 20 times they will fail horrendously at doing it. It's capricious, it's frustrating, it takes agency and choices away from the PCs when they really should be in the spotlight, and quite frankly, it's no fun.
Choices are not real choices if all of the options present an equally good outcome
This isn't to say that players should succeed at everything they try; in fact, far from it. Choices are not real choices if all of the options present an equally good outcome, the same way that choices aren't real if every option is a bad one. The "Monty Haul" style of game is just another manifestation of how un-fun a lack of choice can be, since when characters aren't presented with real challenges we never find out what interesting things might happen if they fail. The trick is that failure should be something that occurs because characters choose to take on a task that's too big for them and then get outclassed and/or fall prey to bad luck, not just because the capricious finger of fate decided that today was the day for a PC to die. Think about it - if you had to lose a beloved PC, would you rather have her be killed by dueling an opponent who was at her level or higher because she chose to defend her honor, or by being crushed under a rockslide because she botched a Climbing roll on her way to the next adventure? Failure is a great theme for games to explore, but it can also suck a lot of the fun out of gaming when presented in a way where choices are divorced from consequences both negative and positive.
Because of these beliefs about failure, it's my opinion that GMs should examine the critical failure mechanism in games they run and decide whether its existence might frustrate players rather than jazzing up a game with a bit of random fun. In the case of a mechanism that could prove frustrating to the unlucky, consider describing critical failures in a way that makes it clear that they're the result of random bad luck (just like the dice roll!) rather than a personal failing of the character or a moment of inexplicable ineptitude. For example, when possible I describe Firearms botches as a gun jamming and failing to fire, or Perception botches as a target's whispers being drowned out by an unfortunate low-flying plain. It makes botches go down a lot easier when players hear them described as random chance rather than a moral failing on behalf of their character. Things like a character accidentally shooting herself or her friend, or drawing a ridiculously wrong conclusion from what they see or hear, are reserved for circumstances where such an outcome is dramatically appropriate or clearly something that the player wants to explore. (Although Stealth critical failures will always be the result of tripping over a trash can no matter what. That's tradition.)
Beyond examining a game's critical failure mechanics, GMs can uphold the importance of choice in RPGs by getting good at improvisation. An old military adage states that no plan survives contact with the enemy. This is true in the context of gaming as well. GMs should take it as a given that in a group where player choices has true importance, the plans they make for a session will not always work the way they want them to, because there's always the possibility for the players to take things in a different direction. For this reason, when planning sessions I try to think ahead to various courses of action that my players might take when confronted with a problem, and to jot down a few notes about how they'll respond if that's the case. Knowing your group helps a lot with this, since after a long time playing together you can start to guess at the sorts of responses your players may favor and plan accordingly, but any good group of gamers is still likely to throw you a curveball every now and again. So GMs should always keep a clear understanding of the game's setting in mind, try to think one step ahead of their players, and be prepared for anything. (If this is something you struggle with, check out Faking It for my advice on how to improve your improv skills as a GM.)
There may be times when the players really don't have a choice
Of course, there may occasionally be some times when the players really don't have a choice in how certain things occur in your game. This happens occasionally in strongly narrative-focused games like the ones I run and play in, usually when dealing with details of the game set-up. For example, I ran a Buffy the Vampire Slayer game which began with the PCs at their high school reunion when the gathering was attacked by an elder demon. My plan was for the PCs to witness the current Slayer being killed by this demon, with the Slayer's power then being fragmented by the demon and passing on to them - so it would have been quite a problem for the rest of the game if they had succeeded in saving her when they saw her being dragged into the demon's mouth and decided to try for a daring rescue! While the PCs struggled (and ultimately failed) to pull the Slayer from the demon's clutches, the players probably suspected that they weren't going to succeed - but the point is that I allowed them to try, and to feel good about their attempt. Every so often for the rest of the game, players would make reference to "that time we almost saved the Slayer," so I know it was memorable no matter what. Similarly, I've been known to make use of "cinematics" that function similar to a cut scene in a video game, in which I briefly describe a scripted event (usually an interaction between NPCs or the setup for a battle) without permitting input from the players. I make sparing use of these - probably only one every 10 sessions or so - and always make sure to warn players beforehand and make sure they've said or done everything they want to before the "cinematic" begins. So choice isn't absolute in all circumstances, but when it isn't possible, GMs should either provide their players with the illusion of free will or make it clear what aspects of the situation can and can't be affected. (Also, if your games are less narrativist than mine, a problem like this is unlikely to ever come up anyway.)
It all comes down to this: Know your players. Know what they want from the game you're running, and find a way to give it to them. Sometimes, as in the above example, it may mean steering events in a particular way, but most of the time it means letting them decide what they value in a game and what interests them. When players are clearly stating that they've made a choice, give them what they want, and ignore or eliminate anything that gets in the way of their choices. Obstacles should be interesting and dramatic steps on the way to a goal, not arbitrary roadblocks thrown up by a GM or a system. Whether your PCs succeed or fail in those goals is a choice in and of itself...but it's one that I leave up to the dice and you.