One of the questions that perpetually swirls around off-hours Dungeons & Dragons conversations is the correct balance between strict adherence to the rules and the Dungeon Master's authority to change things. Interestingly, this debate goes all the way back to the very earliest editions of the D&D game, when the rules were so skimpy that it was flat-out impossible to play unless and until the DM made a host of decisions about how things would work. Even then, people debated to what degree the DM was justified in altering or setting aside rules that conflicted with or contradicted one another. In fact, an awful lot of what happened around the gaming table was spirited discussions (some might call them arguments) about what this or that rule actually meant, or whether something was a rule at all.
The level of rules writing, developing, and editing has risen tremendously since then. It's a tribute to both the clarity and completeness of the D&D rules that when we sit down to play now, only a tiny portion of our time is spent wondering whether we're doing something strictly according to the rules, and the answer is never more than a page flip or mouse click away.
But being clear and complete doesn't make rules sacrosanct. There's still plenty of room for DM input.
D&D's rules are like that often-maligned "book" you hear so much about in war movies. "Must you do everything by the book?" screams the fiery young lieutenant to the hidebound major. "Good men are dying out there because of your book!"
The implication is clear: people who rely on "the book" lack the imagination and intelligence to figure out a better way on their own.
That, however, is precisely the point of "the book." It doesn't define the one and only way to fight a battle. Instead, it codifies a tried-and-true way of doing something that you've never done before, or you're unsure about, or you don't have enough time to study fully and formulate your own plan. "The book" establishes a baseline of competence for those who might not otherwise rise above that level.
Always implicit in "the book" is the notion that no one can foresee every circumstance that might confront a commander at the scene of battle. If a situation calls for a different answer and a commander is confident in his ability to make that call, then it's expected that he will ignore "the book" and do what's going to achieve victory.
Consider the (somewhat pre-"book") example of the Roman army. Legions were trained to line up and fight a particular way. Their generals frequently had less experience at war than the soldiers themselves did, so when a general was unsure about how to conduct a battle, the best thing he could do was let the soldiers line up and fight the way they were accustomed to–"by the book."
But then a general would appear who had the experience, the knowledge, and the insight to see a better way. Men like Gaius Marius, Scipio Africanus, and Julius Caesar recognized when the surest way to victory was throwing "the book" onto the trash heap and making up their own rules.
The same thing applies to being a DM. "The book" ensures a standard of quality and competence. If you run D&D exactly as it's spelled out in the rules, powers, magic items, and monster stat blocks, you won't go wrong. You will get a game experience that's exciting and enjoyable for everyone, and all of your time around the table can be spent getting drawn into the adventure rather than puzzling over unfamiliar rules.
But the rulebooks don't know your campaign or your players the way you do. As the DM, you're the commander on the scene. Hannibal is waiting for you on the field of Zama, outnumbering you by 3:2 and with 80 war elephants to emphasize his point. Like Scipio, you may need to adapt to the situation by arraying your cohorts in a new way.
The current D&D rules are more robust and flexible than they've ever been. They will bend through amazing angles without breaking, and you know what they say about things that ain't broke. We bend them all the time—witness Dark Sun, Eberron, and Forgotten Realms, all of which have unique adaptations. Your campaign can be just as unique, if you take a lesson from Marius, Scipio, and Julius.
We'd like to hear how you've tailored D&D to fit your campaign. Send your insights to firstname.lastname@example.org