In the midst of the battle, Brutus sends Messala in haste with bills (orders) for rhe legions on the other side (Cassius’s wing of the army). Octavius’s forces appear to be vulnerable, so now is the time to pour all of Cassius’s forces into the attack. Brutus urges an immediate attack.
(NOT IN THE BOOK: While this maneuver successfully overcomes Octavius’s troops, it also leaves Cassius vulnerable to Antony's troops. This illustrates the shifting fortunes of the battlefield and supports the larger theme that human choices often yield unforeseen, fateful consequences.)