Real people can't interact with imaginary things. Interaction is a broadly causal notion, and fictional things don't exert causal influence on real things.
This is why I think we have to talk about the declaration and resolution of actions by participants.
I guess I'm not very clear on the limits of interaction then. My intent was to preserve breadth by not defining away the possibility of GM-less games. That is, games where advancing the state of play is handled by procedure or the like rather than by a living human selecting things. E.g. you used the phrase "engaging with, and generating, a fiction, an imaginary set of things and events, that is shared among the participants". That sounds, to my ears, like a more specific way of saying interacting with a fictional space.
In Apocalypse World, certain action declarations enliven a mechanical procedure - a player-side move. Others do not, in which case it is the GM's job to make a move in response (generally a soft move, unless the player has handed the GM a golden opportunity on a plate).
The quality of the design - if one accepts that it exhibits a reasonable, or even high, degree of quality - consists in the relationships between what sorts of action declarations trigger player-side moves?, what sorts of fiction are likely to be established by the GM's soft moves?, and what sorts of player goals and aspirations for their PCs is the PC build and subsequent play process likely to give rise to? If the relationship between these things gets out of whack, then play will not be very satisfying.
Building off of this (since you know this stuff better than I do): This reminds me of something that stuck with me, both from reading the text itself and from my experience as a player before I became a DW GM. This idea that the rules of PbtA games are meant to be "off" in some sense until something turns them "on," and then as soon as they've done their job, they turn "off" again. Like those funny little machines on YouTube that turn themselves off when you flip the switch to turn them on, except productive rather than merely humorous.
Playing DW can sometimes
almost seem like some of the ways FKR is described. I have gone whole sessions without anyone rolling any dice, spending any resources, or in any other way actually triggering the rules. Well, other than the
End of Session move, which is always triggered by, uh, ending the session. During sessions like that, we just talk through things. What the player wants, what the world needs, whatever makes us happy, set rules-free. (Pun absolutely intended.) As the book describes it, much of playing Dungeon World is like having a conversation with your friends. You don't need mechanics to have a conversation, though there may be rules of a sort for them (akin to the Agendas and Principles, but totally informal.)
But in
most sessions, at least some of the time, the player wants to do something, or the world needs something, that can't be simply talked through. A threat that may come to pass in whole, in part, or not at all. A risk taken. A gesture made. Something
uncertain. For the vast majority of such things, there already exists a move tailor-made to resolve this uncertainty.
Hack & Slash resolves the uncertainty about whether, and how, a player's physical confrontation with an opponent succeeds.
Defy Danger, that most versatile of moves, resolves the uncertainty about whether, and how much, a player may act or endure despite hardship.
Parley resolves the uncertainty about whether (and how much) the player(s) can convince a third party to act as the party would like them to. Etc.
You use the appropriate move because something is happening and needs to be resolved. Once it's resolved...the move is no longer required, because it has done its job. It has told you what you need to know in order to move past that critical uncertainty. Hence why I say the rules are "off" until something turns them "on," and then you turn them "off" again on the way out.
DW's design quality (and AW's, though I have no firsthand experience with the latter) comes in part from making these rules triggers natural, clear, and easy to process, while still having them be widely applicable for the things the given game is "about" in terms of tone and gameplay. It also comes from making the rules themselves, and the Agendas and Principles which guide the "conversation" part of play, really honed to doing exactly what is needed and no more.