I was trying to find the specifics, but there was a doctor who did some experiments during the terror. there was one head whose eyes popped open and looked right at him when he shouted his name, and another head that went into the same basket as his rival that bit the other head on the cheek and couldn’t be loosened for 2 hours.
if the guillotine blade went slightly higher, actually impacting the brain stem, it might be different, although I admit that we are all guessing. personally, if I were going to be guillotine’d, I would request that they replace the blade with a large weight and drop it directly on my head.