Power
They say that with great power comes great responsibility. As an air marshal, you will be responsible for thousands of lives over the course of your career. That's a lot of responsibility, so in theory, this means you'll have a lot of power, too.
But power can be kind of tricky.
On the one hand, you'll (hopefully) be the only one on the plane with a gun. You'll be trained in martial arts and marksmanship, too, so you'll be one heck of a lethal weapon. You'll be the one to leap up and stop evil in its tracks, like Superman—well, if Superman never bothered to change out of his Clark Kent dweebwear.
On the other hand, you will have no control over whether or not the plane you're on suffers an attack. You might be able to stop it, but then again, all the training in the world can't make twenty-to-one odds good. You will have dozens, even hundreds, of lives depending on you to make the right call at the right time, and if you slip up even once, it could all be over.
Ultimately, the amount of power you have will depend on the situation. Maybe you'll have all the power. Maybe you'll be powerless.