They nudge and examine the prey to check for movement. They don´t just slither up to a sleeping animal and start swallowing it without any ado. I guess evolution has taught them certain carefulness.
More carefulness than you think. They examine their prey by constricting it until it stops moving. They are sensitive enough to feel the prey's heartbeat, and will not be fooled by a person playing dead.Well, I don´t know about that. Predators don´t usually spend more energy than necessary. There´s always the balance between energy in and energy out.
)! Or die like a Samaritan! 
