I have experience in this so I'll just chip in.
The first part was the planning. It's not easy, you're both scared but you need money really badly and there's no other alternative in your heard. You know the post office is very badly secured, people don't rob it because they don't think about stuff like that, but you've seen the money there, more than you need, enough to set you up for a long time. There's a guard, but he's not armed, he's old and kind, he won't even suspect you when you get in there.
Then you think about masks, you forget to think about the clothes, clothes that might get you recognized because it's not a big city at all and the post office is very close to where you've lived all your life. You just think that the people you associate with have no business near a post office and a mask is enough. There are no ski masks to be bought anywhere and you don't have any money anyway, your head won't fit in a sock so you take two stockings each and fit them over your head. Two pairs to make sure you won't be recognized. You forget that it's Celsius outside(104 Fahrenheit).
The getaway plan is simple, take off running through a building then jump a few roofs and you'll be safe to drop your masks and leave quietly, you prepare t-shirts to change.
But how to intimidate people? You're not big physically, don't look menacing, voice isn't helping. You think knives are no good because it'll invite people to might to try to wrestle you to the ground or run. You get two guns, at least we have that and they're in great shape, you use them pretty often.
So then it happens.
You wake up, meet up and prepare, guns hidden in a plastic bag. You get behind the post office, there's trees and no people around, repeat the plan and think again if you're ready, you're not but fuck it. You've ran all the way to here and you're sweating, you're tired, but the adrenaline makes you forget that. You pull the stockings over your head but you realize you don't see shit and you're sweating all over the place. Breathing is very unpleasant. You stay a few minutes longer in the bushes trying to makeholes for eyes and mouth. Now it's better.
Game time.
You didn't stake out the place in the last few days but no matter. You run through the door, it's 20feet to the counter where they serve the money or pay up, the cash registers are busy, it's incredibly hot, there must be 30 people inside, you didn't expect that, people are paying bills. The guard has his back to you when you enter, you completely miss him and run to the counter where you scream as hard as you can while shakily taking out your guns "Arms in the air, money on the counter now, you're being robbed!".
2-3 seconds of complete silence, the room is so big that your voice echoes. In the room or inside your head, you don't even know. Everyone is frozen in place "is this really happening?". You yell again with the arms shaking. Your knees are giving up. There's sweat in my motherfucking eyes, I can't see properly, I am completely unaware of the people behind me, I'm just looking at the person in front of me who still has the cash register open.
And the you feel it. A strong slap over the head, you fall to the ground and someone is picking me up. I don't even know what my partner is doing, I don't even know if he's inside, did he chicken out? Holy shit, am I alone here? What was that loud slap and the pain I feel on the back of my head? Was I hit?
And then that old guard picks me up. By the ear. Pulling on my ear until I'm on my toes. "You're in a lot of trouble, what the hell is wrong with you?"
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That's how I got arrested as a 5 year old for playing stick-up in a real post office with my toy gun with a friend. We wanted money for a basketball.