I think it has everything to do with the actors. If you go back a few posts to the The Apartment scene that Billy Wilder describes, watch that clip and look at the actors expressions. There is a huge amount of subtext there (way beyond the mirror bit), and it's all in the facial expressions. I think Wilder knew that his actors would get it. He didn't need to spell it out. The important thing is that you don't want to spell it out in words, because words fail to express what is going on: the deep emotions and thoughts racing through their heads. You have to write a page or two to express all that in words, and it would ever be as good. Film is visual, not lexical (in words). Images have much more information (a picture is worth a thousand words).