Position of the Centurion

[Zaknafien]

well I agree somewhat with that, but it was the optio's position to keep the century moving forward, the centurion was more of a combatant commander akin to a platoon leader or first sergeant in today's terms i believe. Part of the criteria for selection centurions was their 'hardness' and willingness to fight to the death. their casualty rates exceeeded that of other officers because they were the 'front line leaders'. The centurion I think would obviously stand beside the standard bearer, the symbol of the century and the icon by which the men knew where to stand and for what to fight. it would make sense to me, then, that this would be up front, where men could see it and 'be inspired' by it and him.

[Atilius]

Concerning the temperment of the centurion, Polybios says:

And they wish the centurions not to be so much bold and adventurous, as men with a faculty for command, steady, and of a profound rather than a showy spirit; not prone to engage wantonly or be unnecessarily forward in giving battle; but such as in the face of superior numbers and overwhelming pressure will die in defence of their post.

[Geoffrey S]

I'm half wondering if the OP is too influenced by the RTW depiction of officers.

[antisocialmunky]

I would imagine that the placement of the centurion would be besides a unit that has not been engaged, behind a unit that is engaged, or infront leading a charge: Besides to issue commands to form up or move here and there, behind to determine the best course of action for his unit, or infront to lead the unit by example.

I would refer to more recent formation fighting tactics(pike and musket era) to determine the centurion's best placement since command and control for then was pretty much the same as in Roman times.

[Warmaster Horus]

If I understand Atilius' reference to Polybius correctly - and I'm not saying I do - then it has nothing to do with the question at hand. In the face of "superiour numbers and overwhelming pressure", the sacrifice of the centurion's life is that which is expected of him. Because by then, he has learned the idea of the chain of command, and if his orders are to defend an area, then he must defend it, giving all he's got. And if he doesn't, then he's good for the chops.
About his position, he's supposed to lead by example, like Zaknafien said. And, anyway, if the back row of a century doesn't hear what the centurion says, isn't the optio supposed to repeat the order?
In any case, once the battle is joined, one doesn't really have a choice in who to target. The two battle lines merge together, making it impossible to distinguish one person from another, in the enemy ranks.
When you've got a big, hairy guy in front of you, just screaming for your blood, you can't go after the one who killed your friend. No, you kill the one in front of you, and then go after the other one.

[Zaknafien]

well lets remember that the century was primarily an administrative subdivision of the cohort, which was THE tactical unit of the day. Centuries would not fight by themselves but as part of the cohort. its not like modern-day squads in platoons, the whole platoon would be fighting. the centurion himself, while primarily responsible for training and administrative details, was a leader expected to show the other troopers how to act and respond during battle and march.