At our airline (Canadian carrier), there is no requirement for someone to be seated in the emergency exit row as there is no requirement written into our aviation regulations. As it happens, I find I deadhead more often than not in the emergency exit because no one is seated there, and our pilot contract specifically calls out the emergency exits as priority if there are no open seats in business or premium.
So how to we fill those exits then if no one wants to pay the fee to sit there?
In the event of what is called a Planned Emergency Landing (defined as a non-normal or emergency landing where the FAs have 10 minutes or more to secure the cabin), and where the emergency exit seats are empty, our FAs will identify and move Able Bodied Passengers (ABP) during their pre-landing procedures. In the highly unlikely event that something happens last minute and the FAs cannot move an ABP (called an Unplanned Emergency Landing), then it's up to them to manage the situation after the aircraft comes to a stop if they were not able to move someone before landing. However, given it's one of the first steps in their Emergency Landing Checklist (ELC), at most they'd need 3 to maybe 5 minutes from when we told them to grab their ELC. Gallows humour as it may be, there are not too many emergencies that develop quicker than that where the state of the aircraft after coming to a stop does not provide passengers and crew with a newly formed emergency exit.
As it applies in Canada - and I'd assume the regs are the same in most other jurisdictions - here is an overview of the regs that apply to passengers and the emergency exit (there are a bunch more about sizing of the exit and crew training, but those are not applicable to this discussion):
1) All emergency exits and any associated equipment (e.g., slides) must be operative.
2) Any person who is seated in an emergency exit must be briefed on the use of the exit and when to and when not to use it. If that person declines the briefing or is unwilling to perform the duties, that person must be moved.
3) That no child restraint system is permitted in an emergency exit row seat.
4) That access to the emergency exit is not blocked (for example, by power cords, bags, loose items of clothing, etc).
5) That the person who is seated in an emergency exit will not adversely affect the safety of passengers and crew during an evacuation.
To meet this last requirement (#5) my last few airlines would teach the acronym PINCODE: Pregnant, Infant, Non-English Speaking, Children, Obese, Disabled, Elderly. If anyone seated in the emergency exit fell into one of those groups, it was up to the Cabin Manager to determine if they thought the person would adversely affect an evacuation. If so, they would be moved and told how to obtain a refund. If the Cabin Manager did not think they would affect an evacuation, they would be left in the seat.