Was the roper introduced due to the Action Declaration of the player or due to the success with the Perception check?
Or in other words, what would have happened if the player had rolled poorly - would the roper have been introduced?
That's an excellent question, but I can't recall properly. At the time I must have had something in mind, but I don't know what I would have done - maybe a surprise attack from the roper?
I think I was using the Monster Vault (as opposed to a hardcover MM), and I have the vaguest of memories of looking at the Perception DC to notice the roper, so that suggests the roper was on my mind after the action declaration (looking at the stalagmites) but before the roll was actually made.
Something more colourful than that, but more or less. The table was in no doubt as to what had happened - the player in question had thought of the possibility that there might be a roper disguised as a stalagmite, and I had been prompted to look up my book (as above, I think in the context of calling for a Perception check) and then narrated the roper. When my friend was reminiscing about it a few weeks ago, he was the one who was laughing at it being the (other) player's fault, as it was the (other) player who had introduced the possibility by declaring the action.
My view is that in classic D&D that sort of GMing would be outrageous hosing of the players; but in 4e it actually makes for a more fun game - the players can see the GM upping the stakes, and can make their decisions in response, and 4e players have a remarkable depth of resources to respond with and the recovery of resources is linked in part to the XP-weight of an encounter (via the milestone system).
I don't know if I've ever heard the GM described as using Force. I typically reserve Force to either physics or threats/use of violence. But you know what? I'm down. What kind of Force do I use as a GM? Why Magnum Force of course!
This is a ridiculous inference and a pretty outrageous imputation of motive onto me that is utterly unsupportable. Seriously.
We're simply talking about the concept of work; energy transferred to an object by way of an application of force. In TTRPGs, its the GM applying force to the gamestate and/or/both the attendant state of the fiction in a manner/by a means which subordinates player's or system's rightful input.
Now someone might say "a system has no rightful input so a GM can't suspend system's rightful input" or "a player has no rightful input so a GM can't suspend a player's rightful input." Effectively saying "every input onto play to propel it is the purview of the GM and the GM only and that GM can then choose to give (or veto) system or player input at their discretion." Yup, that is a thing a person could say to deny that "GM Force (covert or overt)" is a thing. But now we're having a fundamentally different conversation in totally different territory. Pretty damn controversial territory so I hope the person making that statement is prepared for some pushback from a whole lot of directions in the TTRPG-osphere.
"Force" in the sense being used here comes from stage-magic jargon. A "force" is what it's called when a magician presents a mark with the illusion of choice or random chance — "pick a card, any card!" — but they already know what the "choice" will be, because they only allow the mark to draw the 7 of diamonds.