Keep banned items from entering the country.
An airplane contains a tiny community of travelers who agree to follow the rules, behave rationally, and avoid hurting one another. When passengers decide to break that community pact and act out, an Air Marshal steps in to enforce order.
When you’re an Air Marshal, you may wear rumpled clothing and look tired and irritated in order to blend in with the other passengers when you board the plane. You take your seat and maintain your cover, all while being acutely aware of the other passengers around you. If a passenger becomes violent, makes threats, refuses to listen to Flight Attendants, or displays some other form of disruptive behavior, it is your job as Air Marshal to break your cover and step in.
You may move and handcuff the passenger to another seat. You may also direct the Pilot to land the plane if the person refuses to comply. In extreme cases, you may be forced to use your handgun to quiet the passenger, although you’ll avoid this as much as possible.
Of course, you spend much of your time in airplanes. This may be a good way to save money on rent, since you won’t need to keep up a fancy apartment, as you’ll rarely be at home. If a major threat is detected and you’re needed to monitor a flight, you may be given only an hour’s notice.
Sometimes, you’ll be asked to provide security for train systems and other modes of transportation as well. In such cases, you wear your best suit and display your credentials on your jacket. You may wear dark glasses and a stern expression, too.
You want to look prepared, confident, and just a little scary. Your very presence may deter people from even contemplating an attack.