Per an Airbus training document, “The selection of ABPs is based on their ability to understand instructions, their physical ability, and their ability to stay calm”. Naturally, priority is given to off-duty airline crew members, and police/fire/military personnel. It makes sense: they are already trained for certain emergency situations. A Flight Safety Foundation article warns that some emergencies happen so quickly that there is no time to hand-pick an ABP – the flight attendant will simply select a nearby passenger.
What does an ABP do? It depends on the configuration of the aircraft and where the ABP is sitting. If it’s a standard door, a crew member may open it and inflate the evacuation slide. Then, two ABPs may jump out first, hold the evacuation slide and assist passengers as they leave the plane. But in some aircraft and with some exits (such as the Boeing 737 overwing exits, which are nowhere next to a flight attendant jumpseat), or if a flight attendant is killed or injured, an ABP will be responsible for opening his or her exit, while the other ABPs perform crowd control (blocking the exit until it’s ready for use, then directing passengers to use it).
In the United States, FAA regulations require passengers sitting next to an emergency exit to comply with many criteria which can be summarized to two things: ability to understand instructions and physical ability. Airlines will publish those criteria either on their website (for example Southwest Airlines) or on their safety cards.
So, do you think you would make a good ABP?