This is more about mercenary warfighters than the regular Army-O. I'm going to be drawing from my research and comments I received from various servicemen here, so expect the details to sound somewhat more prosaic than the flashy SF romances recommended above.
Whether you're in some ground pounder infantry Division or assigned to a Cavalry unit, a merc is a whole different animal to the kind of soldier who draws the King's shilling.
For one thing, the government soldier only draws a shilling. The mercenary gets paid in guineas.
A character becomes a merc because his ticket stipulates a large sum of pay for his services, possibly with bonuses for particularly outstanding work - which, in this case, may mean "saving the hiring company a fortune."
The Company, or Consortium of companies, hiring the merc unit has one objective for your unit in mind - to further the company's / companies' interests.
The thing is, no matter how much you and your unit get paid, or how well-equipped, you are the cheapest, most economically-viable option available. Every operation your character goes on is done with the bottom line in mind. Waste no ammo, or as little as possible. Save whatever resources you have, because the bean counters at the top will be checking and if you go over budget, they dock it from your shares at the end of the contract.
Pretty quickly, mercs learn a most important lesson from this. Don't use up your own ammo. Kill a native, steal his gun and use up his ammo instead.
And that's basically it. Sign up for a one year contract, watch your back, do as you're told, go where you're told, try not to get yourself killed, keep your head down and then come home to the missus driving a posh car.
Just don't expect there to be anyone weeping over your fallen comrades on Veterans' Day, because you're not a veteran. You're just a merc. And there's no such thing as Mercenary Memorial Day.