How to show powerful emotion in a character trying to hide it?
This question deals with showing the emotions of characters when those characters are actively trying to hide their emotions. If the emotion is something subtle, like apprehension or annoyance, you can easily convey the emotion through things like body language and the way the character speaks, or the things they do. They realize they're breathing quickly, they glance nervously around corners, they clench their fists, or they storm through doors. The character might not even be aware that they are emoting.
Little hints like this work for subtle emotions. But they do not work as well for powerful emotions. Ramp those same two emotions up to, say, terror and rage, and simply showing quick breathing or a clenched fist isn't going to get the message across to the reader. There must be a larger hint, a more obvious display of the emotion, for the reader to get just how powerful it is. There has to be a moment where the character's emotions break through, if only for a moment, and he really shows them (running in terror, yelling in rage, etc.).
The problem comes when the character in question is actively trying to quell or hide that emotion. You can get away with subtle hints, because the character might not realize he's emoting. But to show more powerful emotions, you need more obvious hints, and it's hard to believe that the character will emote that way and not realize it. He will stop himself before he starts.
Here's an example:
The war between the vampires and the elves has raged for many years. Our hero has seen first hand the suffering the vampires are capable of inflicting, and has felt loss at their hands. Young and head-strong, he is consumed with rage towards them. Our hero joins the war, and throws himself into every conflict against the vampires. His father, fearing for his safety, joins his platoon, and watches him closely. Then one day, our hero, blind with his rage and thirst for revenge, throws himself against several elite vampires. Alone and outnumbered, he quickly realizes his mistake, but it's too late. At the last moment, his father comes out of nowhere and slays the vampires, but takes the deathblow intended for his son. As he dies in his son's arms, he is at peace, telling our hero that he 'kept him safe'. Now realizing what his endless rage against the vampires has wrought, our hero swears to never let it blind him again. He returns home to his family, but his rage follows him, refusing to be quenched.
In this example, the hero feels an undying rage towards the vampires, but at the same time, he's now afraid of what that rage might cause, and makes every attempt to quell or ignore it.
I might be able to show simple hints, like a clenched fist or storming out of the door, but those won't convey the vast hatred this character feels, and the character certainly won't allow himself to give the reader any more hints than that.
Question: How can I show powerful emotion in a character who is actively trying to hide that same emotion?