Felix wrote about his colonoscopy. It sounds terrifying, even though he wrote at the end not to worry about it. He also didn't write about any "butt stuff", which I'm also not concerned about.
Basically, you don’t quite go “under” like you would for things like surgery. Twilight still enables the patient to be responsive to verbal commands, and can communicate (I think). But the thing with Twilight is that the patient has no memory of the procedure. The drug inhibits the creation of new memories.
Honestly, in addition to the process itself… the administration of this drug was also fairly unsettling to me. I heard stories of people emerging from this having said or blurted out all manner of crazy things.
I had no idea this drug was involved, or that it had as creepy a name as Twilight.
Anything can happen to you while the drug is active, and you won't know it.