I made this guide because I noticed many MPs struggling with the problems mentioned (and fixed) in this guide. I will also keep it as short as I can, so people have enough strenght to read it.
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The biggest mistake many MPs make - not using dicretion correctly
Your job is to keep the order and enforce the safety of the personnel in Sulaco. You don't always need to use force to deal with every situation, words are usually much more effective. In fact, directly tazing and cuffing someone with out explanation will get the marines hate you - and after that it's even more challanging to do your job. Use the force only if the marine continues trespassing/disobeying for example commander's order even after your warnings. You are much more likely to not earn "asshole" reputation amongst the marines. Try to approach your suspect with negotiation intent if it's possible, sometimes it's not but in many cases it is.
Your relation to command staff
Sometimes the commander might contact you and give you instructions. Follow what he says, but remember to remain lawful in your actions. If the command staff requests you to arrest someone and gives no charges, ask them for charges before arresting the suspect. You need to know the charges when you are arresting someone, because you need to tell your suspect their charges after you've arrested them. Usually the command staff doesn't order an arrest on anyone with out reason, but you should stay vigilant and think everything for yourself. If the commanding officers give you confusing orders which make no sense, ask them to clarify their order.
You should remain polite and appropriate, especially when talking to command staff. You are an example to be followed, so give a salute when higher ranked officer passes by. If you have nothing to do, ask the commanding staff if they have any tasks to give you. If you are lucky, you might end up getting a task to take care of the survivors.
Your relation to the marines
Now the second biggest mistake I've seen when it comes to interacting with marines, is giving them an impression that you are dominant, strict and not approachable. If you think you are the law, you should probably change your job. You are nothing more than a staff sergeant, the same rank as squad leader, but you have different tasks than a squad leader has. Do not think that you are above anyone, you are just here to follow and enforce the commanding staff's orders. You are also to assist marines if they are confused. If you can see a confused marine running around the hallway, ask him to go to briefing politely.
Commander gives an order to get to the shuttles - you enforce it. If the marines hate you for your bad behaviour, don't expect anyone to listen to you. As has been said earlier, you are an example to be followed. Ask the marines to follow the commander's order - and usually they will. If the marines show signs of decent behaviour, you have no reason to get angry at them. The best MPs have a reputation of being relaxed. Marines see that kind of MPs much more comfortable to listen to.
Extreme situations
If the situation gets totally out of hand and it's not your fault - it's time to use force. Sometimes there are totally horrible marines who cause chaos by running around the briefing room just for fun, break windows and run around in inappropriate clothing. It is likely that this kind of marines will not stop their crimes until you stop them by force. Remember to follow arresting procedures and give them a sentence mentioned in marine law for their crimes. Remember prisoner rights so they can't complain about anything. You should still remain appropriate when handling the criminals, then you will have nothing more to lose.
All in all
Being a MP requires specific social skills, it's not enough if you just know your equipment and the marine law. You must also know how to interact with your "customers" as well. Remember to use discretion!
TIPS
Wickedtemp wrote:Emphasis on the whole "If Command tells you to execute someone for petty theft, or gives an equally outlandish and illegal order, you probably shouldn't do it." thing.
"I'm just following orders." isn't an excuse. After the round, Command will get arrested and the MP that carried out the illegal order will meet the same fate.