There is a paradox to D/s activities that misty's original post and foxy lady's post point out--consent is necessary but the whole point for this slave (which is what i think of myself as) is to get into sub space and be under the dominance of my Mistress and/or Master. It has seemed to me that the best experiences i have had have been those where i was taken to places that i had darkly fantasized about but never expected to actually go. i had shared those with my Mistress but identified them as "just fantasies". I never gave actual advance verbal "consent" to those activities and when She introduced some of them into a scene, She did not ask me first if i objected. However, i was so deeply into subspace at the time that i just went with it and every time was glad i did.
Two important points, i think. First, when She did these things (not all at one time but over a period of time) i had already been serving Her for a while (we were not in an exclusive relationship with each other), i strongly trusted Her even though somewhere in the back of my head was a voice saying "OMG this is going a lot further than i thought it would." Second, i do not feel i ever lacked "agency" in the sense that we had a safe word which i knew, based on my trust of Her, She would honor. But when i get into sub space, i am not ever really thinking of safe words or "time to stop now". i fall into a mind set of obedience and submission, even where i might be begging not to do something and sort of mean it, but do it anyway because Mistress has ordered it.
So i am not in disagreement with Misty's opening post in principle, but before this thread developed with very good posts by everyone, it did sound a little like one of the pc policies of "consent means there must be explicit verbal consent before each additional step in a sexual encounter is commenced". i do not mock those policies but i think they would be utterly unworkable in a D/s scene that would be at all satisfying at least for me.