Dean tries to turn his fear into something funny and it's not scary at all.

agree with natalie88, just that time in yellow fever, you could see dean being scared and it was still funny ;-) :D

agree with you both :)..dean hides his feelings well...sam just couldn't...but still Sam does it better than in the earlier seasons when used to go all out abt almost evrythng.

I surprised myself by picking Dean! It's his eyes...when he started having flashbacks from Hell, those eyes were completely terrified. The same goes in the last scene of Season 3 when he's suspended from the chains and calling for Sam...totally chilling!
With Sam, it's more the facial expressions - which he does very convincingly!

I don't know if it's the way their faces are shaped or a credit to their acting skills, but Jensen and Jared are good at the way they play their characters, even when they're terrified. Sam looks so scared, but Dean always looks like the tough, nothing-ever-scares-me guy, even when he's frightened.

Yes, like 67impala said its in his eyes,