But wouldn't that then mean that anything or everything the GM does, without prior express player consent, in essence, be railroading?
No, because there are ways of not acting unilaterally that don't depend upon express consent.
Here's just one simple illustration, from 4e D&D play: a player builds their PC, and plays their PC, as a fanatical devotee of the Raven Queen; and so I, as GM, introduce undead, and Orcus cultists, as threats to the PC or to things the PC cares about.
That is not unilateral: the player is the one who has made the Raven Queen vs Orcus/Undead conflict salient. But it doesn't depend upon express consent.
Upthread,
@chaochou talked about
who authors the players' goals for their PCs. And I talked about
the players exercising real influence over the significant content of the presented scenes, and their stakes, and what follows next. Those aren't exactly the same, but they're in the same general space. In my illustrative example, it is the player who - by way of their build and play of their PC - is shaping content and stakes.
To provide a fully-worked illustration, more would be needed. For instance, if the whole scenario is a GM-orchestrated "fetch quest" and the GM just replaces
goblins with
skeletons then, while the player has influenced the
colour of the opposition, it is still the GM determining the stakes. And so I would still see that as pretty railroad-y. It becomes non-railroad-y when "threat to the PC or to things the PC cares about" is also responsive to player-established priorities. Which comes back to
@chaochou's point about *who picks the PCs' goals?"
How then could a GM function as a member of the group with the power to add to the ongoing narrative? The players are entitled to add to the narrative unilaterally, shouldn't the GM have the ability to do that as well without it automatically being labeled railroading? I mean, how is a GM supposed to function, on a fundamental level, if all instances of narrative manipulation are automatically considered to suppress player agency and be labeled as railroading?
As I posted upthread,
GM presents and players interact is pretty fundamental to RPGing (except at the avant garde cutting edge). But that doesn't mean that all RPGing has to involve the GM dictating what happens.
There are plenty of ways of the GM adding to the fiction that don't involve the GM unilaterally deciding on the significant content of scenes, the stakes of scenes, and what comes next.