As a Dungeon Master running games of Dungeons and Dragons I am often faced with the dilemma of do I use rules as written or the rule of cool. In most cases I side with the rule of cool. This rule is, if the players have an idea that there isn’t really a rule for or might even be kinda against the rules I will wave the rule in favor of the inventiveness of my player. The one time rules as written is my go-to is in spell casting.
Players using spells in ways the rules don’t normally allow is probably due to the players lack of understanding of that the spell does. In D&D spells can get dense and reading them all the way through can become tedious. In my experience players, especially new ones, will simply skim the text of a spell or just look at the name. I know this because I will be asked questions about what a spell does when the text of the spell very clearly says what it does. The most common example of this is new players will select Mending thinking that is a healing spell. From the name alone this makes sense but in the body of the spell it very clearly says that it can only repair inanimate objects.
Having players read their spells all the way through can usually help mitigate any conflict, but not always. The real challenge arrives when a player has read the spell and wants to make a small tweak. An example would be, a player wants to use Poison Spray to make puffs of poison in a room to see if they can find the location of a someone that was invisible. The problem is that the first line of the spell reads, “You extend your hand toward a creature you can see…” If the player can’t see the invisible creature they don’t fulfill the requirement to cast the spell and thus the spell can’t be cast.
Scenarios like this is ultimately where the conflict arrives. Do I say no they can’t cast the spell because they don’t have a target for the spell or do I say yes because it is an inventive use of the spell. I want to encourage creativity and engagement. In a story telling game this wouldn’t be a conflict, because the rules of the spells are much more loose. In D&D the rules for spells are far more rigid and well defined. Ultimately, as a DM, it is a good idea to know what you would do when faced with such a scenario, not just for spells but any action the players might take. In the same way you would prepare for a session have an idea of what you want to do but be prepared for the players to do something you don’t expect.