You've said this a few times so let's explore what it actually means.
If I am saying my characters does X how is that action not rooted in the fiction? I'm also not quite sure what a non-genre appropriate PC action looks like. Now I think you mean something more specific or nuanced by that statement but I can't tell what it is.
Without knowing what the established fiction in your example or what action X is, I can't blanket answer your question. This isn't being coy -- what's grounded in the fiction for a given action is directly related to what's currently going on in the scene and what's already been established.
For instance, if it's been established in the fiction that Bob the NPC is blind, and a PC declares they show Bob a picture, then the outcome is that Bob can't see the picture. No roll is available for that action for which any outcome is possible -- showing a blind man a picture cannot make the blind man see the picture. However, if the PC leverages some means that fits the fiction to cure the blindness so they can show the picture, presumably using magic or medicine, then that action is grounded in the fiction in that it acknowledges the blind NPC as part of it's declaration.
I'm going to carry this last example into genre appropriateness. If we're playing a D&D game, then the action to cure Bob with magic is genre appropriate. An action to cure Bob with medicine is not (it's not in the genre understanding of non-magical medicine to be able to cure blindness). However, if we're playing a sci-fi game, then magic is not genre appropriate, but medicine may be (depending on tech levels and whatnot). If the sci-fi setting includes magic via 'sciency' powers, well, then, magic is back on the table as genre appropriate, couched in the proper terminology (psionics, nanites, whatever).
These aren't things that I would consider terribly difficult to grasp. They're not an attempt to carve out anything special. Grounded in the fiction simply means honors established fiction. Genre appropriate simply means that the action makes sense in the genre of game you're playing. These aren't high bars for most players, who are going to do this normally without prompting. I keep bringing them up in these discussions to forestall people from outlandish examples they think match the adjudication methods I'm discussing.