While I was already aware of how western art until roughly the XIXth century showed ancient historical events or biblical stories in an anachronichal way (such as romans wearing plate armor), or their extensive disregard of proportions, I always wondered why medieval art, especially in its earliest form, showed every character with a standard, smiling face instead of actual situation fitting expressions. This especially is flagrant when it comes to tortured biblical or historical characters, or battles where soldiers are butchered, which seems to endure any form of torture with perfect calm. Some instances :

http://blogs.getty.edu/iris/files/2013/09/gm_00403601.jpg

https://fr.wikipedia.org/wiki/Jeanne_d%27Arc#/media/File:Vigiles_du_roi_Charles_VII_10.jpg

https://s-media-cache-ak0.pinimg.com/236x/30/90/76/30907658a25d430642638dc55ec17f09.jpg