For those of us who have been DMing a long time, the answer may seem obvious (and most of the responses were in accord), but there is some nuance to that question that requires a bit of analysis for our younger, newer
There is a famous quote by one of the original authors of Dungeons & Dragons:
“The secret we should never let the game masters know is that they don't need any rules.” - Gary Gygax
You see, the rules and mechanics are there to provide us a framework for telling the story… but they are not the story and should not necessarily dictate narrative. The rules are not what make the game fun. What makes the game fun is the collaborative storytelling and epic moments produced by the players and DM working together. But, what’s really important, and not stated in the original question is fun. Why do we play these games? To have have fun… Otherwise, what’s the point?
Caveat: There is a difference between the game as a whole being “fun” versus any given moment of the game being “fun”. A favorite character dying can be a bummer, but the experience that lead up to the character death may have been the best time ever… And sharing memories with your friends about that time Dave’s PC epically died in a heroic (or decidedly unheroic, but humorous) way can be great fun, even if, at the time, it was a bit of a downer.
Given the above stated assumptions, the “mathematics” of D&D is basically: Fun > Story > Rules
Almost always, but again, there is nuance (and caveats).
Don’t Break the Rules
The rules provide a scaffold of consistency. Players expect the game world to to work they way the rules say it should work. When a player creates a character, there are certain expectations on how their skills, spells, feats and what not will work within the game. Because players have only a sliver of control on the game world itself (in D&D, that is), it is important cherish the sanctity of their control by not changing aspects of the rules that affect their characters. [For other games like FATE and Dungeon World, the players have more narrative control over the larger world, but that is beyond the scope of this article.]
Also, when you make an exception to one place in the rules, you may be opening a door to allow for that exception to apply to other similar circumstances. Your players may press you to use the same exception in those situations and you will have to delineate why it applies in one scenario and not another. Let’s face it -- players are always looking for that tactical edge. If you always stick to the Rules As Written, you can easily say “Sorry. That’s not what the book says.” (I’ll tell you why you shouldn’t say that below… but let’s not get ahead of ourselves).
The rules not only provide consistency for the players, it also provides the ability to you as the DM to put your foot down when the players are pressing for some unreasonable or game-breaking rule compromise they want. However, that does not mean you should never bend or break the rules...
Learn the Rules before you Break Them
For those of us who can still remember high school creative writing, there was an oft-quoted maxim used by teachers in the U.S. “You must first learn the proper rules of English grammar before you learn to break them.”
|Where exactly are those mounted combat rules?!|
A deep understanding of the rules is needed so that when you do choose to break the rules for the purpose of story or fun, you do so in a thoughtful and meaningful way. You do not want the game to devolve into “Let’s Pretend” without real structure or consistency. Also, as noted, players will often look for an edge or loophole and may press you to change a rule to fit their desire.
As an example, I often see GMs ask about “called shots” in D&D… Like you might want to blind a foe, or disable their sword arm, or shoot them with an arrow to the knee. The problem with this kind of request is that it circumvents the abstractness of hit points. D&D doesn’t support called shots to a body location because it bogs down play speed and breaks the combat abstraction represented by Armor Class and Hit Points. However, the Sharpshooter feat does allow ranged attackers extra damage if they take a negative on their attack roll. Or the Battle Master can use a Disarm maneuver to remove a weapon from a foe.
Without that intimate familiarity about how the rules work, you might be tempted to allow a player an exploit that breaks the core mechanics, rather than use existing mechanics that can simulate the same circumstance… and I guarantee the players will use that called shot constantly if you make it as powerful (or more) than the Sharpshooter feat already allows.
If you are uncertain of the long term effect of a house rule will be on the game, that may be a time to rely on the Rules As Written. However, you could offer the player an olive branch in the form of “I’ll allow it this time, but if this ends up being an over-powered or abused house rule, I reserve the right to revoke or change it at a later date.” Which leads me to...
When You Should Break the Rules
|Up the rigging, you monkeys!|
Players play D&D and other RPGs to do Awesome Things. They want to swing from a a pirate ship's rigging like Errol Flynn while fencing multiple opponents. They want to parkour off the wall to leap over a foe. They want to shoot orcs in the face while using a shield as a snowboard. Your job as GM is to help them do these Awesome Things. This is often called the “Rule of Cool”... The gist is that if an idea is cool, you should allow it to occur regardless of the rules as written (with the caveat that it shouldn’t be a player abusing the rules, or an utterly unrealistic action within the confines of the setting/genre).
This brings us back to the Fun > Story > Rules equation. When a situation that comes up that is outside the Rules As Written, ask yourself a couple of questions.
1 - Will it advance the story in an interesting direction or in a way that will give the player a feeling of investment in the story? Then you should probably say yes, and break the rules.
2 - Will it enhance the fun or create an awesome moment for the player(s) to shine at the table? Then you should almost certainly say yes and break the rules.
Example 1: In a Pathfinder game I ran for my nephews, the PCs were chasing down a gargoyle through city streets. One of the characters went to the rooftops and as it tried to fly away to escape, the player asked. "Can I jump off the roof onto its back?"
Now, we were playing on the grid and I quickly glanced at the relative positions of the minis... and he was going to be a couple movement squares short to actually jump off the roof. My response, "Yes, absolutely. Make an Acrobatics check."
Rule as written, a DM could easily say "No, you can't move that far"... but that would be pretty lame. In the "Theater of the Mind" play style, there would be no question that the DM would respond "Yes." It's a cool idea, could make for an interesting story twist, and is going to be hella fun. So why not just allow it on the grid even if you bend the movement rules a bit? When you think about it, even movement on a grid is a highly imperfect abstraction for combat positioning. Let the player have that moment of awesome. Trust me that you won't regret it.
Example 2: In another comment on my blog, a player asked the DM, “Can I use my Dragonborn breath weapon (cone) to shoot at the floor to damage creature surrounding me?” [as opposed to one side or another based on the standard cone template]. The DM said “Yes, but you will also be placing yourself within the area of effect.” That’s a great ruling for a creative idea, even if it’s not RAW.
Example 3: In another from my home games, the Rogue player asked to use his Disengage bonus action and parkour-leap over the foes (using the terrain that was available) to move to a spot that would normally be blocked by movement-through-enemies rules. I responded, “Absolutely, but you will need to make an Acrobatics check.” After the game, the player actually thanked me for "letting" him do something awesome. I considered it a pretty minor request, but was gratified that it created a really memorable moment for him in game.
|I believe you have something of mine...|
Remember Fun > Story Too
This has been written about tons and tons already all over the internet, so I’ll try to just summarize for the new GMs out there.
The “story” is also less important than the players’ actions and their effects on the game world. There will be many times in your games where the players will do something that will completely short-circuit the plot… or they might possibly even sail right past the current plot onto something else that captures their attention. They may kill the villain before events you had planned for him have take place. They might accidentally stumble upon the solution to a mystery causing you to lose a couple sessions worth of content. Or they might want to ally themselves with the pirate leader instead of fighting for the King.
Let it happen.
If we go back to the gargoyle example I used earlier, perhaps the story (as written in an adventure module, or planned by the GM) calls for the gargoyle to escape. If the player succeeds in leaping on the gargoyle's back and bringing it to the ground, that's awesome! Perhaps it will complicate the story line and the GM may have to improvise how the adventure changes as a result... That's OK! That's more than OK... It's awesome! The players are supposed to create fun, heroic moments and if that "screws up" the story, so be it.
Their fun is more important than your story. It’s not your story, anyway. It is owned by everyone at the table. So if they do something that completely thwarts the plot you may have planned out, don’t punish them for being clever or even lucky. Don’t force them back onto the rails. Rethink how their actions will change the active plot. Let their actions have an impact on the world, and the fun will take care of itself.
Story trumps Rules.
Fun trumps both.
Say yes and enhance the awesome.