The little "frown" seen on the PT clone troopers has always struck me as representing the trooper's mouth. However, ever since I'd first seen a stormtrooper, I always seem to view it as their nose. If it's the mouth, then it's confusing, as the helmet seems to go lower than the clones' helmets. I always thought the very bottom of the helmet was the mouth... It just seems more awkward than the clone helmets. Which is it?
Pretty sure it's supposed to be a mouth-analogue; it's even got teeth. As for the odd proportions, there were some liberties taken when transferring the original design into a physical prop. Here are some concept designs for what stormtroopers were "intended" to look like: [image=http://www.originalprop.com/blog/wp-content/uploads/2008/06/ralph-mcquarrie-stormtrooper-concept-art-ii-star-wars-cropped-x425.jpg]
[image=http://www.plasmainfusion.com/SW-153%20Stormtrooper%20Helmet.jpg] Obviously it's intended to resemble a mouth.
Come to think of it, they look pretty smug - as if they're laughing at some stupid joke, like Beavis and Butthead. Stormie 1: Hey, TK, did you hear the one about the Jedi and the Twi'lek? Stormie 2: Yeah. That's a good one. Hehehe. Stormie 1: Hehehe. Stormie 2: Hehehe. Stormie 3: Hey, guys, what's up? Stormie 1: Oh, nothing much. Hehehe. Stormie 2: Hehehe. Stormie 3: Hehehe. Vader: You three! Come with me. Stormie 2: Yes, mylord. Stormie 1: What's our mission, mylord? Vader: You are to disarm a bomb in the Senate. Stormie 1: Very good mylord. Stormie 3: Hehehe. Stormie 2: Hehehe. Stormie 1: Hehehe. Vader: Would you mind explaining what's so funny? Stormie 2: Oh, it's nothing, mylord. Just a little joke. Vader: I see... Stormie 1: Hehehe. Stormie 2: Hehehe. Stormie 3: Hehehe. Vader: Hehehe. Stormies - they are smug /LM
On the subject of unsettling masks in the movies, one mask that really bugged me was the Ishi Tib: [image=http://nd01.jxs.cz/458/706/799be5a638_13855569_o2.gif] At a glance, you would think those dark holes in the center of the face were the creature's eyes, but upon closer inspection, you'll see that the eyes are on stalks above the head, and those holes are actually the Ishi Tib's nose.
