For example, did they (ostensibly) have to go through a sort of 'checklist' before attacking a target (like today's Western armies; verbal warning, warning shot, minimum application of force etc)?
Or was it as straightforward as "See target? Go for it"?
EDIT: Thanks for the answers everyone! The only downside is that, for whatever reason, you're compelling me to go on an 'ancient Greek' reading binge. I feel like I've barely scratched the surface with the Romans and already you're distracting me!
At least during the Republic, Roman generals cared a lot about omens. There's a famous story during The First Punic War over a Roman officer being furious over not being allowed to engage the Carthaginians because the Sacred Chickens refused to eat. And in the end it looked like the chickens were right.