Incomming!
I am that guy that operates best under nasty circumstances. When everybody else is in full panic, I flatten out and do what's necessary.
When to not be calm? I have used anger to make a point more times than I can think of. When I was coaching Jr. High football I once threw my clipboard down and screamed FUCK at the top of my lungs. Strictly against the rules and should have gotten me fired, but I had every lineman's full and complete attention for at least 10 minutes.
Nobody said a word about my indescretion :)
That's another thing to look at.. apparently it seems as though a show of chaos can get people to take you much more seriously.. Sometimes having a calm demeanor doesn't always get people's attention but a sudden burst of anger and everyone is alert and everyone is listening..
Human nature is a weird thing. And finding and controlling balance is.... Well.... Unpleasant
At the end of the day when it comes to interaction... Humans react to humans.. We've probably all dealt with these feelings so if someone displays a burst of anger we can probably relate to it and imagine or at least know that there must have been something causing it...
It won't always have the desired effect ofcourse but recognizing the emotion goes a long way..