Fiona, I don't really think we can see everything he is thinking from his expression.  We can take guesses, and some of the guesses may be right, but they are guesses.  
I guess a more accurate way of me putting it would be, we can see what he wants us to.Whether that particular expression is a true reflection of what he is or was actually feeling, you are right,we will never know.I think I have now seen most of his films and he  looks and sounds so different in them all.
I am sat now watching the day after tomorrow,on T.V  and Jake  looks younger, but still looks like Jake, same with Rendition,he even sounds the same.
I think I am struggling to convey what I feel. 
I just think Heath's  face and expression was so chameleon like.
I also loved his eccentricity, formal suit with sandals, or big scuffed boots.Or shorts with formal shoes.Maybe it's the Brit in me, we like our eccentrics.