Armor
Classes
Conflicts
Ethos
Flaws
Glossary
Kits
Maleficence
Origins
Shields
Skills
Spells
Stances
Status Effects
Tactics
Talents
Techniques
Treasure
Weapons
"Jack of all trades and master of none" is a great way to describe a potential GM. Running a game requires improvisational skill, curiosity, empathy, imagination, emotional labor, vision, and patience.
Even so, there's no need to feel overwhelmed at this: the role might require a hodgepodge of talents, but you don't have to be a pro at any of them to put together a decent game—you just need to be willing to try.
As Referee
Your primary function as GM is to choose the best mechanic for the job when the outcome is uncertain.
And when the rules-as-written don't provide a clear answer, your secondary responsibility is to make a fair decision in the spirit of the rules.
As a referee, you need to be consistent, fair, and firm.
As Director
The vast majority of the time, you'll be narrating, either to set the scene or to describe how the fiction evolves after the PCs take action.
You'll also be managing the spotlight.
In the real world, the spotlight means which player you're having a conversation with in the moment. Within the fiction, the spotlight refers to whatever the "camera" is focused on. As you direct the narrative, you want to keep rotating the spotlight to different players as much as possible, so that everyone gets an equal opportunity to shape the fiction through the conversation.
As Author
You are also the author of the adventure, whether you're running a pre-written OSR+ module or an adventure of your own design.
The difference between a GM and an author, however, is that you don't know how the story ends. Instead, you write a rough outline of what the story is about, and instead of constructing a plot with linear scenes to support that outline, you present a bunch of scenes for players to discover in a nonlinear way. Your chief concern as an author of the adventure is to incorporate your players’ session zero story hooks into those nonlinear scenes. You also connect other events in the fiction with the PCs’ narratives, depending on what they decide to do. This drives the game towards a conclusion, since the objective of play in OSR+ is for players to resolve their PCs' conflicts.
This overall process is covered in great detail in the section on adventure design.
As Guide
The real-world needs of the game extend beyond the fiction. You've got to wrangle your players and get the game scheduled, prep for each session, set up the physical or virtual environment, and even sometimes mediate conflicts between players.
Figuring out how and why your players play is key to managing your table. Think of yourself as a guide rather than a manager of these things—someone who’s there on equal footing to help them get together and achieve their goals.
As Player
Last but not least: you play.
The GM is still a player in the game, albeit a very special one. You should be enjoying yourself as much as your players, and it's easy to lose sight of this after all the refereeing, guiding, authoring, and directing.
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Dungeons & Flagons