I've rambled before about what a genius Stephen Colbert is. One of the best aspects to his character is how he maintains his strict face.
Except when he doesn't.
Here's a clip from his correspondent days for The Daily Show. He does his thing and gets some pretty good laughs. Then he starts to crack up. Soon it gets to the point where you can see the pain of him trying not to laugh, which of course makes it all the funnier. But by the end he's got it under control again. It's such a treat to watch.
On a smaller scale, here he is unable to keep his face straight the entire time as he presents "Stephen's Laws of Love."