[IV.153] betimes: early. Formerly 'betime'; "the final 's' is due to the habit of adding '-s' or '-es' to form adverbs; cf. 'whiles' (afterwards 'whilst') from 'while.'"—Skeat.
Act V
[V.1] The plains of Philippi: Capell | The Fields of Philippi, with the two Camps Rowe | Ff omit.
[V.2] battles: troops, battalions. 'Battle' was used for an 'army,' especially an army embattled, or ordered in battle array. The plural is here used with historical correctness, as Brutus and Cassius had each an army; the two armies of course coöperating, and acting together as one. Cf. 'battle' in l. 16 and 'battles' in [V, iii, 108].
[V.3] warn: summon to fight. Cf. King John, II, i, 201. In Richard III, I, iii, 39, we have "warn them to his royal presence."
[V.4] am in their bosoms: am familiar with their intention.
[V.5] bravery: bravado, defiance. The epithet 'fearful' probably means that fear is behind the attempt to intimidate by display and brag. Dr. Wright interprets 'bravery' as 'ostentation,' 'display.'
[V.6] bloody sign. "The next morning, by break of day, the signal of battle was set out in Brutus' and Cassius' camp, which was an arming scarlet coat."—Plutarch, Marcus Brutus.
[V.7] Plutarch tells that Cassius, though the more experienced soldier, allowed Brutus to lead the right wing. "Shakespeare made use of this incident, but transferred to the opposite camp, in order to bring out the character of Octavius which made Antony yield. Octavius really commanded the left wing."—Clar.
[V.8] exigent: exigency. So in Antony and Cleopatra, IV, xiv, 63.