Would you agree that RPG with absolutely no player input or rules would reduce to the activity of one person telling a story?
Yes. This is one of the reasons why I don't have a particularly positive opinion of the typical way the "traditional GM" role is presented. Because that's exactly what it looks like to me. Player input is irrelevant outside of coloring within the established lines. You get to decide if your roses are red or white or yellow or maybe green if you're feeling
adventurous.
In the case of one person telling a story my understanding is that it is relatively well defined and accepted what "diegetic" means?
Sure. I also don't really consider it to be a
game anymore, so there are no mechanics.
That is it should be a reasonable test for a proposed definition of diegetic in context of TTRPGs, to see if this definition reduces to match the well established definition in that edge case?
Not really. By having taken it to such an extreme, you have removed it from the space of games and firmly planted it in the space of stories.
I have not really grasped your suggested definition of diegetic, so could you help me understand it better by explaining how it would be evaluated in light of the above?
Well....as noted, it can't. Because I don't think the above
is a game anymore.
For something to be a game, as opposed to a story (no choices, you just experience it) or a puzzle, you need:
- Multiple (2+) genuinely distinct choices, which potentially lead toward (local*) victory conditions and loss conditions
- Relevant stakes will be set, and (local) victory or loss conditions will preserve/enhance them or harm/weaken them respectively
- The players are able to make reasonably-informed decisions about which choice to take
- The players can learn, from the consequences of each choice, which choices are better or worse to make in the future, meaning, the consequences that arise from these choices are solely the function of what choice the player made, the information they genuinely could have known (even if they failed to actually seek/discover it), and the rules of the game (including dice randomness)
A campaign which railroads players--visibly or invisibly--
dramatically harms these elements, sometimes different ones depending on the specific technique used (e.g. the fourth point is severely harmed by illusionism, while the first point is severely harmed by any form of railroading). It is uncommon to cause
so much harm that any given point is completely eliminated outright, but plenty achievable to cause so much harm that the result is only barely a game at all anymore within large chunks of the experience.
By these lights, something that goes as far as you have described genuinely exits game territory entirely. It is no longer a game. It is a story being experienced ("witnessed" is the term I've used previously) by the players. The GM has control over a great many things, and thus needs to exercise that control extraordinarily carefully. Changes to the rules or the input information should not happen--ever--during a time when the players are making decisions on the basis of that information,
unless the players are given a real, reasonable chance to learn of the change (meaning, as I've said before, no "you must get three nat-20s in a row" BS--this needs to have a fairly good chance of success, if it isn't said outright.) Hence my previous example of the GM eliminating fudging by making their intrusion into the game-space diegetic: it's not a secret manipulation of the game concealed from the players and sustained as a pretense of a consistent world with consistent rules, it's a detectable, understandable, potentially even quantifiable and
controllable change, the discovery of a new rule that new choices can regard when they're being made.
So...I can't answer the question as you've asked it. The thing you have presented isn't a game, given what I think a game needs to be, so the answer is "whether it achieves diegesis is not relevant to
games doing so".
*"Local" here because people hate saying you can ever "win" anything at all in D&D, even though that's ridiculous. You can win combats. You can "save the day" and thus successfully complete a "quest". You can acquire the treasure you sought, or clear your name, or whatever. These are local win conditions. They are things that might happen or not happen, where you desire them to happen. Likewise, local failure states abound.