So how do you go about playing a Security Officer successfully? Well the answer to that question lies in explaining what exactly a Security Officer is.
A Security Officer is Diplomat, Policeman, Warrior and Peace Keeper. It is not the job of Starfleet Security to romp about the galaxy blowing the enemies of justice to hell. It is vital that you remember that the primary role of the Security Officer is preventative in 99% of cases. If it comes to combat you've likely done something wrong somewhere.
The particular stations that each individual officer will hold are largely dependant upon the environment in which they are stationed:
Starbases: Starbase Security Officers are the true Policemen of the trade, spending virtually all of their time patrolling sectors, handling the mundane matters of Security Administration and preventing or investigating crimes, both big and small. Obviously on a station such as Starbase 1 you will have virtually no crime at all, and you'll spend most of your time looking pretty and standing still. However on the frontier outposts such as Outpost 42 in Fleet 3, Outpost 45 in Fleet 2 and OED V in Fleet 6 you are distinctly more likely to come across crime. You'll even spend time creating some of your own arrests, particularly during character development stages. Having served as Chief Of Security for Outpost 42 myself I can assure you its certainly interesting work for a Security Officer to undertake.
Science Vessels: I'll warn you now that serving aboard a Science Vessel can be damnably boring work, and it takes a special kind of player to be able to successfully portray a Security Officer on a vessel like this. You will spend the majority of your time, even during a sim, writing posts that show you training, performing various mundane tasks such as patrolling areas, standing guard, performing Brig duty and running emergency action drills. A science vessel post isn't for everyone, and when the action does arrive, as it inevitably will once the weenies have had their fun you'll need to be prepared to handle whatever your de Sade wannabe GM may throw at you. Science vessel Security Officers will be given the opportunity to really exercise their diplomatic muscles, particularly when it comes to missions involving other cultures where you will be expected to guard your shipmates whilst not offending your hosts.
Combat Vessels: There has been a lot of debate as to the inclusion of combat vessels in STF, but I think anyone who has ever served on one will agree, they are quite possibly the most fun you can have with your clothes on. Combat Vessel Security Officers will have access to the best equipment, the best armour and weapons that Starfleet can provide, because nine times out of ten they'll need to use them. Frontline vessels such as the Draco in Fleet 5, the Atlantis in Fleet 3, the Endeavour in Fleet 5, and others will almost always receive combat related sims. But! And there is certainly a But here, this is actually not necessarily the kind of ship on which you spend your time firing phasers and hitting the dirt with photon mortars. You will spend your time on a Combat Vessel carefully balancing your innate desires as keepers of the peace against the potential combat situations in which you will find yourself.