Tactics
Officers are expected to provide a bit of tactical insight, or at least that's what the handbook says. Truth is, the sergeants, with their greater wealth of experience, tend to do most of the practical work. Where officers come into their own is in dealing with other officers. They may have additional, non-military areas of expertise on which to fall back, or may improve their soldier's morale if they enjoy a certain amount of popularity in the ranks.
Lead from the front.
An ideal officer has the best interests of his unit at heart and should want to see to it that they perform well. An efficient unit reflects well on its leader. He recognises his unit's strengths and shortcomings and adjusts his plans to suit. When in doubt, he keeps his plans simple: 'charge!' is often best. If he wishes to improve morale, he takes his turn at sentry duty, makes sure he knows his troops by name, but draws the line at making a habit of fraternisation.
Talk the talk.
Officers might know their way around a sword or lance, but their true skill should lie in their tongues (get your mind out of the gutter right this minute) — coming from privileged beginnings, they should be skilled at negotiation. If captured, the lower ranks look to them to lead escape attempts and provide a role model. They must appear cool, confident and unflappable.
Know your enemy.
The higher orders can be pretty cut-throat, and those who went to boarding schools no doubt have a few volumes of horror stories about brutality to share. It's all preparation for life, however — they should know how to drive a hard bargain and to second-guess enemy officers. Dealing with meddlesome heroes falls in the same category. Are the PCs expecting to sneak into an area around the back? The officer should anticipate this, make educated guesses at the infiltrator's capabilities and concentrate forces and firepower accordingly. They should ascertain the most advantageous terrain, and know how best to put equipment to use.
Negotiate
A good officer might have to make sacrifices when needed, but he should not throw away his life or those of his men recklessly. Depending on the situation, retreat or surrender may be advisable, and if he wishes to preserve the lives of his soldiers he must be prepared to resort to diplomacy. They must be able to put their enemies at their ease, appear reasonable and honest — and know the exact moment to attempt to escape.
Know your sergeant.
I've alluded to this a few times already, but let's get things down in black and white. The officer gives the orders and his sergeant makes sure those orders are carried out. The sergeant should in theory defer to the officer's rank, but the officer defers to the sergeant's experience. The sergeant has a hundred and one fiddles going on and knows the rulebook inside out — that's his weapon against the higher ranks. He's the shield between the officer and the rank and file. The officer should expect this, and probably allow it if the platoon performs well — however, he should keep his eyes open and at the first sign of failure be sure to remind the sergeant who's in charge and demonstrate an even greater command of the rules. The key to this potentially antagonistic comradeship is discretion: both participants are involved in the army game. Both are playing for authority, but there are unwritten rules that must be followed, and both players lose if the unit begins to suffer.
Know when to turn a blind eye.
A good officer is aware of what goes on in his platoon, regiment or company, but again discretion is called for. Hounding one's men for the merest infraction, continually punishing them under every obscure rule in the book ultimately hurts morale. Yes, it's good for the troops to know where they are with you, but if 90% of that time they know they're in the glasshouse, chances are they're not going to rate loyalty particularly highly. Draw the line when it looks like the unit may be compromised.