Seriously, his fists always seem to be closed while he's standing. I'm only basing this on the Kurosawa films I've been seeing lately.
But I wanted to quickly mention this scene from Throne of Blood that I just loved as it highlight's Mifune's intensity and ability to convey emotion through facial expression.
After he and his companion Miki encounter the forest spirit and are on their way to receive accolades for defending their Lord's realm, there's a great moment when Mifune's character Washizu walks toward his ruler to receive his reward. I love the look on Mifune's face, full of seriousness and intent, as he walks forward.
Contrast that with the look on his face after receving the reward when he is promoted according to the forest spirit's prophecy. Now, the look on his face – as he walks in the opposite direction – is full of fear and doubt.