Like
@Umbran above, my position on illusionism is that it's unavoidable.
Well, I tend to think of "illusionism" within the context that it was introduced. From
here, we first get a definition of "Force" which is then used to explain "Illusionism":
Force: the final authority that any person who is
not playing a particular player-character has over decisions and actions made by that player-character. This is distinct from
information that the GM imparts or chooses not to impart to play . .
Force techniques include [action declaration] manipulation, fudged/ignored rolls, perception management, clue moving, scene framing as a form of reducing options, directions as to character's actions using voiced and unvoiced signals, modifying features of various NPCs during play, and authority over using textual rules. . . .
Force Techniques often include permitting pseudo-decisions . . . Also, Force Techniques do vary in how flexible a scene's outcome is permitted to be. Some GMs (to use the classic single-GM context) might do anything up to actually picking up your dice for you in order for you to talk to "that guy," or he might let the characters miss the clue, either 'porting it to another character or letting its absence go ahead and affect the outcome. . . .
Illusionism is a widespread technique of play and arguably, textually, the most supported approach to the hobby, as testified most recently by the publication
Secrets of Game-mastering (2002, Atlas Games). It relies on Force . . . . GMing with lots of covert Force is called Illusionism. I call that the Black Curtain; if the Curtain is drawn, then the players aren't immediately clued in about the presence and extent of the Force itself.
Force (Illusionist or not) isn't necessarily dyfunctional: it works well when the GM's main role is to make sure that the transcript ends up being a story, with little pressure or expectation for the players to do so beyond accepting the GM's Techniques. I think that a shared "agreement to be deceived" is typically involved, i.e., the players agree not to look behind the Black Curtain. I suggest that people who like Illusionist play are very good at establishing and abiding by their tolerable degree of Force, and Secrets of Gamemastering seems to bear that out as the perceived main issue of satisfactory role-playing per se.[/indent]
The very second a GM "extrapolates" something from existing notes + player actions + knock on effects of player actions, the GM is basically engaging in "illusionism" --- he or she is creating new in-fiction notes / factors / game states which are unknown to the characters and will remain unknown to the characters unless the characters engage in a very specific set of actions --- actions ultimately defined by the GM --- to uncover the revised game state(s).
If the extrapolation occurs via the narration of consequences, or the framing of a new scene, then the new "game state" is not unknown, and does not require a specific set of actions to uncover.
This is how some RPGs make Force, and hence Illusionism, impossible.