Where would the Centurion stand in a battle line?

[M. Caecilius]

The account of Spartacus engaging two centurions at once in his final battle is a likey confirmation of this.

Good call. I wonder whether the famous episode mentioned by Piotr, the rivalry of Vorenus and Pullo, could also be used. Unfortunately, this occurs during a siege (the Romans being besieged). Do we know whether the legion was drawn up in battle order behind their rampart, placing these two side by side?

If I'm not mistaken the mortality rate of centuriones was very high. And that would place them right in the harm's way. The account of Titus Pullo and Lucius Vorenus by Julius Caesar doesn't show centuriones as especially timid.

The casualty figures of centurions in the Caesarian armies and those of his contemporaries are extraordinarily high, and the accounts of their sometimes daredevil actions explains that. Pullo and Vorenus are perfect examples.

But Polybius explicitly mentions that the Romans "wish the centurions not so much to be venturesome and daredevil as to be natural leaders, of a steady and sedate spirit" (6.24.8 ).

It looks like there was a change in attitude between Polybius days, and those of Caesar. I wonder when and why this did occur.

Signum will be definitelly not in the first rank

What do you base your certainty on?

I believe this was discussed before, in particular with the debate on who the antesignani were. By the etymology, they would have been troops specifically selected to stand in advance of the signum, indicating that the regular troops stood behind or level with it. The (very late!) command of the Strategicon of Maurice "nemo antecedat signum" ("let nobody march in front of the signum") could confirm this, if we can accept the huge gap in time between these sources.

Frontinus (Stratagems 4.5.3) mentions Postumius leading a charge whilst bearing a standard, although that was an exceptional situation. Similarly, Sulla seized a standard and placed himself in front of his army (Appian, Civil Wars 1.7.58). Several Roman commanders tried to encourage their soldiers by throwing their signa into the enemy ranks, suggesting that the standard bearer was close enough for the commander to snatch his standard (Frontinus, Stratagems, 2.1-5 mentions Servius Tullius (thus likely apocryphal, but to work, the situation itself must be possible), Furius Agrippa, T. Quinctius Capitolinus, M. Furius Camillus, and one Salvius).

The execution of the standard bearer who hesitated to attack the Faliscans (ibid.8 ) would suggest that he was to lead the charge, much as the aquilifer leading the landing in Britain did.

On the other hand, Caesar, in Civil Wars I.43 speaks of a retreat "to the standards (of the legion), indicating that the standards stood somewhat removed from the action.