You are right @profgreen. There is usually, or always, knowledge of what will happen next so you can mentally prepare. And that is important.
i have been in a film situation where an action was suddenly sprung on me – and I said no. In hindsight, I would have agreed to do it had it been mentioned beforehand so I could think about it.
not always the case – or sometimes things go wrong or too far. But then, as an actor, you have the knowledge that you could potentially burn the take to stop whatever happening.
an important part of an experiment is that it should be the naturalistic reaction of the subject. And that is impacted if you know beforehand.
so to be valid then the experiments have to be unknown to the participants, but the experimenter would obviously wish to know that the experiement might be a success and so have a degree of knowledge about the participants.
so the example of being tied to a chair – equally valid if someone allowed it or if no one allowed it. But a waste if that was the first step in an extended experiment that could be totally wasted if the first step failed.
knowldge, and therefore “scripted”, is more necessary if the aim is to have a product to show others (ie a film).
and Ed like this