Game design is making others play in ways they wouldn’t play on their own, without being present. The game then is your means of making others play in a certain way.
This concept is important for understanding the various types of content in an RPG book and their use. Those include:
- Procedural rules: Do this…, then do that.
- Advice, guidelines.
- Choices for character traits, equipment, spells…
- Character sheets and other fill in forms.
- Tables, maps, flow charts and similar visualistion methods.
- Pictures of characters and locations.
- Maps of the game world / setting.
- Tokens and other playing materials.
- Ready made PCs, NPCs, monsters, locations, treasures, adventures.
- Signature characters. Named and archetypal example characters, that may be used in…
- Short stories, at the beginning of the book or chapters.
- Quotes from other media, often at the beginning of chapters / paragraphs.
- Lists of other works that inspired this game.
- Fictional table dialogues: A written interaction that might happen at the gaming table.
- Actual play videos.
Not all games use each method and there are probably some I forgot. The important thing is that each can contribute to have the players play in certain ways and you can use several of them in concert. For example if you want to make a game of hunting monsters, you will probably include some advice on making the monsters and some ready made monsters. But there are other still some other aspects. Is it a comedy something more gritty? What’s the tone? You can throw in some lists of inspirational works, pick proper pictures and throw in some media quotes. What is the typical structure of a monster hunt? You can use some procedural rules and a flow chart to symbolize it. Or rely on advice and an example scenario. Or you do both.
One very powerful tool is relying on other games your players have played. There are some very short RPGs, a page or two. Those cannot possibly work on their own. They work for people who have played other RPGs in the past and can provide all the missing pieces themselves. The more you deviate from what people likely know, the more work you have to put in to make that game work. And the more of the channels from the list above you likely want to use.