Salvete,
A nice example of how the leaders of the army interact with the soldiers is given by Caesar in the second book of his De Bello Gallico. When the soldiers were still busy building most of the camp, they were suddenly ambushed by the Nervii and Caesar describes:
"...Caesari omnia uno tempore erant agenda: vexillum proponendum, quod erat insigne, cum ad arma concurri oporteret..."
"...By Caesar everything had to done at the same time: the < red > flag had to be risen, which was the signal, for when it was fitting to run to arms"
This vexillum was a red flag which was placed on top of the commander's tent as a signal for an upcoming battle (or an ongoing battle in this example
)
But during a battle the most important means of communication were the Aquilae, as Marius pointed out before. Each soldier knew what sign to follow. The trick of course was to get the right commands to each of these sign-bearers in time.
Valete,
Lupus