Someone upthread suggested "linear" means "pre-determined". As a general rule, making it up as you go along doesn't involve pre-determination, especially if many of the constraints on what can be added (eg because elements of the gameworld have already been stipulated) are introduced by the players in the course of play...
If, as the game progresses, the GM makes things up in response to the players' choices, so as to foreground elements and themes that the players bring into play and to background elements and themes which they don't, where is the railroad (as in, where is the vitiation of player choice?) Undoubtedly exploratory choices (like in the secret door example) aren't being validated, but then we're not talking about exploratory play here, are we?
It took me a very long time to realize that "making it up as you go along" has multiple meanings. Granted, not as many meanings as "story" to roleplayers, but enough differences to cause confusion in this kind of discussion.
For example, what it means to me is usually not some form of, "pull X out of my subconscious as a quasi-reaction to player A having his character do Z." It seems that is what some people mean, though. Nor does it mean, "treat the player A choosing Z as a choice of what he wants explored, so I'll react with Y, which seems thematically appropriate." No, what it usually means for me is, "I don't have something definitively prepped for player A doing Z, but I do have very clear ideas of how this world works and what the relevant NPCs think and plan. So in that context, I'll have them do K--
which is what I probably would have chosen had I thought to prep this material absent any player input."
It is a somewhat "simulation" technique, and obviously different from what most people mean as, "making it up as you go along," which on the surface would normally seem to be the ultimate "anti-sim" technique. Yet, this is so important to me, that I think of it as a tool for "setting integrity," and the players in the game can rarely tell whether I'm working off of prewritten notes, ad hoc decisions, or a mixture.
When discussing, "making it up as you go along," the antecedent of "it" should be precise.
Related, per the Champions disadvantage example, I'll note that we sometimes ran rather traditional (gaming style) Fantasy Hero games where the disadvantages were deliberately somewhat vague. Instead of every character being Hunted by someone they had ticked off prior to play, we gave every character a 20 point generic hunted on the not unreasonable expectation (based on the nature of the players) that they would be ticking off someone on a regular basis. Note that this is not a change of characterization or players having narrative control, but a way to manage a game where most of the interesting things happen in play.
So here we have an explicit flag on the character sheet, negotiated as part of the explicit contract before the game starts, that we will, "make up some antagonists as we go along."
