What's the matter?
In real life we tend to hide what's really going on, but in improv we want to show exactly that.
If your character is angry, play an angry character. It's really as simple as that.
Of course, if you're a more established actor, you can set a character on stage that's trying to hide its anger, but always make sure your partners know that your character is angry.
Remember, you are both actor and character on stage, but so are your partners. Don't hide information from them, they won't get it.