Levels of physical fitness, drunkenness and attitude are among the "triggers" that cabin crew assess within the first four seconds of greeting a passenger.
There's a lot more happening behind that seemingly benevolent smile and standard welcome greeting given by cabin crew to passengers as they board a flight.
Creating the impression that they are "warm, approachable and competent" is only one element of the greeting, according to Gaea Peregrinor, an American who has been an air hostess for 25 years.
"I'm evaluating you very closely," Ms Peregrinor revealed on Quora, an online Q&A forum. She explained that she memorises the faces of any passengers who seem to be physically strong and could be of help in the event of an attack on board.
"If a situation looks like it could develop, I'll privately and discreetly ask one of these people if they would be willing to help us if necessary.
"Help might involve subduing or restraining an unruly passenger. We hope it never happens, but we will prepare just in case it does," she said.
Equally, cabin crew also make note of any passengers with any injuries, disabilities or deficiencies that would disqualify them from sitting in an exit row, as they would need to be able to lift a heavy hatch or door, while passengers who aren't fluent in English might not be able to follow any commands or read the instructions on how to open the exit doors during an emergency, she added.
Fliers who appear intoxicated are unwelcome on board as the "potential for future problems is too great", while those with a "hateful and nasty attitude toward the crew" need to be "addressed before departure. It's rare but it has happened," she added.
"I've had passengers board who look pasty and pale, deathly ill. We removed them; nobody wants their flu germs!" she added.
But passengers who are also airline employees are considered an "invaluable resource", particularly those with a knowledge of in-flight procedures.
"They've been trained in what to do in an emergency, whether medical, mechanical, etc. They know how to handle the situations as well as I, and are trained to become an instant 'team member', fitting right in immediately if needed," she said.
One such passenger was vital during the 1989 United Flight 232 crash landing in Sioux City, Iowa, "a disaster that should have killed everyone on plane" but didn't with the help of another pilot on the flight whose assistance in the cockpit "helped save so many lives" she recalled.
"Consider that air travel is fraught with inherent danger, made more so by the political and religious climate of the world today, one must be constantly alert and aware of one's situation. So when I greet people, you better believe that I'm always very aware of each passenger who steps through the door of the aircraft".
"The whole idea is to prevent problems from getting airborne, and be prepared for them if they do develop in flight," she added.
Other alerts on air hostesses' radar include passengers who attempt to smuggle pets or bottles of alcohol in their purses, handbags and briefcases.
"Booze is allowed as long as it stays capped. You just can't drink your own liquor on the plane," she added.
While greeting passengers as they board a plane might require a lot of concentration, the thoughts of flight attendants while saying goodbye are less urgent.
"As for thanking people as they leave, I'm probably thinking about getting out of my uniform and relaxing in the layover hotel, or at home! Or, I may be trying to figure out if I have enough time to grab a sandwich on my way to the next flight".
Last year, airline cabin crews revealed their biggest in-flight bugbears, from being handed used nappies to passengers walking around barefoot, in a list published by Flight Attendant Career Connection.
Other air travel irritations included taking forever to decide which drink you want, leaving rubbish in the seatback pocket or using the bathroom right before landing.
The Telegraph, London