Follow up from What if there are no volunteers to sit in the emergency exit row? and @mins 's comment:
I'm curious... What prevents someone to say they will assist and then just rush out of the plane when the need to assist is here? Tons of people are ready to promise anything to get what they want, and then just say they didn't understand what they were asked for (or more likely swear that they were not asked anything... show me the evidence...)
..What are actually your responsibilities in case an actual emergency occurs and you have to do the assisting tasks?