This rule is simple. If in doubt make sure you react.
You don’t have to ham it up (but sometimes it helps) but as long as you’re reacting to that sword blow, thrown insult, magical wizzy wozzy, strange gas or situation then you’re probably going in the right direction. If you’re unsure, think about what your character might do and wing it. Don’t just stand there like a wall waiting for a special call or a direction, take the situation you’re given an improvise.
In turn if you hit someone, cast a wizzy wozzy spell or otherwise do something awesome – don’t always expect players or monsters to react how you imagine they should (in your eyes). You don’t know their character background, if they’ve misread the situation or are having a crappy brain fart moment (happens to us all). Give them a prompt if you think they need it or nudge a ref gently if you think they’re straying into ‘taking the piss’ territory but remember ultimately we’re here to have FUN and part of that means not everything will go how you imagine it should.