Flight attendants can bar you from flying if you've been drinking, if you're ill or if you're an anxious flier.
When we are greeted by flight attendants as we board a plane we normally just see smiling, welcoming faces.
But are they simply happy to see us or are their brains working in overdrive to assess what kind of traveller we'll prove to be?
Two cabin crew members have taken to US-based online forum Quora to discuss what they notice about passengers when they first set eyes upon them, from whether they've been drinking to how physically fit they are.
Are you intoxicated?
You may think that the pints you drank at the airport might go unmissed but cabin crew can be quick to notice the whiff of intoxication.
Sjaak Schulteis, a flight attendant for 30 years for Lufthansa wrote on Quora: "Do I smell something? Alcohol? Drugs (one can smell Marijuana)... If a guest coming aboard is drunk or intoxicated by any drug, it can happen that he or she is not allowed to enter the plane."
He added: "The first impression is often the right one and we do refuse passengers who might be a danger for the safety of that flight.
"So far I have refused four passengers and was luckily backed up by the purser and captain. All of these were drunken passengers. Three men and one woman, over a period of 30 years."
How physically fit are you?
They're not just admiring your muscly physique for aesthetic reasons - you could in fact prove highly useful in an emergency.
"If I see someone who is muscular, powerful, strong, physically fit, I memorise his/her face and make a mental note of where they are sitting," said Janice Bridger who has been a flight attendant for several airlines for 27 years.
She added: "I consider this person a resource for me. In the event of an attack on the flight or on me, these are my 'go-to' people. 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."
Do you look ill?
Germs can spread easily in the cabin and if flight attendants are concerned about your health you may find yourself barred from flying.
Ms Bridger said: "I've had passengers board who look pasty and pale, deathly ill. (We removed them; nobody wants their flu germs!)"
Do you look like an anxious flier?
Flying can be highly stressful and cabin crew are keen to help allay your fears if they can.
"I often see passengers who are afraid of flying and need a word of comfort and encouragement," said Ms Bridger.
Meanwhile Mr Schulteis eases travellers in by making an effort to speak their language.
He wrote: "We pay attention to their language (check if they hold a magazine or newspaper), citizenship (in Japan I greet with Konnichiwa [hello] or Ohayogozaimasu [good morning]), in Thailand, sawasdeekrap, and so on.. if I know it is in their language I try to welcome the guests and hope I guessed right."
Are you an airline employee?
You might think that on your day off you might be excused from any responsibilities, but if you have any knowledge of a plane and the cabin you could be a useful asset if things go wrong.
"I try to learn if we have any passengers who are airline employees, particularly crew members who have been trained in the in-flight procedures," explained Ms Bridger.
"These people also are a resource for me. 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.
"They are an invaluable resource for me, and I like to know who they are and where they're sitting."
Are you trying to smuggle anything onto the flight?
Amazingly some passengers have tried to smuggle their pets onto planes, the cabin crew reveal. And don't try and drink your own alcohol on board either!
Ms Bridger said: "I've had people try to smuggle pets in their purses or handbags, bottles of booze in their briefcases. (Booze is allowed as long as it stays capped. You just can't drink your own liquor on the plane.)
"So yes, I need to be vigilant and aware, all behind my 'greeting face' of smile and pleasant, comforting welcome!"