A comment in chat piqued my interest:
do zombies blink?
If not, why not? And what adverse effects may they suffer from not blinking?
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The Walking Dead basically abides by Max Brooks' rules in World War Z and The Zombie Survival Guide:
Ghouls don't blink, I don't know why. Maybe because they use their senses equally, their brains don't value sight as much. Maybe because they don't have as much bodily fluid they can't keep using it to coat their eyes. Who knows, but they don't blink and quislings do. That's how you spotted them; back up a few paces, and wait a few seconds. Darkness was easier, you just shone a beam in their faces. If they didn't blink, you took them down.
- "Joe Muhammad", World War Z
Todd Wainyo attributes the milky appearance of zombie eyes to their failure to blink:
I centered my sight between her shrunken, milky blue eyes . . . you know it's not really the eyes that make them look all cloudy, it's actually tiny dust scratches on the surface, thousands of them, because Zack doesn't make any tears. Those scratched-up baby blues were looking right at me when I pulled the trigger.
- "Todd Wainyo", World War Z
Executive Producer and Special Effects Director Greg Nicotero explicitly instructs the zombie extras not to blink; any inadvertent blinking is cut in the editing room:
I directed the episode in which a walker killed Dale. I was very specific with him. He had really good bone structure and he literally looked like a horrific doll. It was a really neat makeup my guys did. In editing I made sure to cut out any instances where the actor blinked. When people blink, that's an involuntary reflex, so with zombies, I try to take those lifelike performance bits away. You watch it and it's so weird, and you might not realize why.
Speaking about the same scene:
You know, the guys did a great job on the makeup and even when I edited — all the close-ups, even when he blinked, I edited out the blink. So he looks like this grimacing skull with these giant eyes, and you know when he jumps on Dale, the shots that you see of him super close, it really feels like he’s right there. So this was really a great opportunity to make sure that you realized how dangerous these were.
- Here's Your Fix
Since walkers really don’t blink a lot, and their eyes are already dead and rotted looking, we built a prosthetic that that built the performer’s faces out and we put fake guys in those prosthetics [to create Michonne's pets]. So, when you’re looking at them, you’re not seeing the actor’s eyes. You’re seeing fake guys because we were able to simulate the missing jaw and the pulled out teeth.
The unusual appearance of zombie eyes and its connection to their failure to blink is hinted at in this quote from Nicotero, in which he explains how zombies on Fear the Walking Dead differ from The Walking Dead:
They have to look relatively harmless until you get closer and then you start looking at their eyes and you start noticing that there’s some wounds or some dried blood on them or something like that.
I look at the eyes like rotting eggs in that the longer that they sit around, the longer that those zombies walk around, the more decomposed and disgusting the eyes get. But at the beginning, the eyes look a bit more like just hemorrhaging and sort of cataract and that’s what the eyes in the beginning of the ‘Fear the Walking Dead’ look like.
- Tech Insider
In short, zombies on the shows are limited to doing what they are told, and nothing more. Aside from blinking, they are told to take shallow breaths to hide the movement of their chests, and refrain from making noise of any kind.
“Look at that, Bob.” The Governor nods toward the little zombie. The child-thing cocks its head, staring at the Governor with a vexed expression. The thing blinks its eyes. A faint trace of Penny Blake glimmers behind its eyes. “My baby’s still in there. Aren’t ya, Honey?”
- The Walking Dead: The Road to Woodbury
Actors playing walkers were told not to blink. According to Addy Miller, who plays Summer, a.k.a. "Little Girl Walker":
Q: Do you have any interesting or funny stories from your time on the set?
A: Well the first one that pops into my mind strangely is about Frank Darabont again. We were shooting the part of the scene where my back is to Rick Grimes and I am turning around. I then had to hold my eyes open and not blink as I started walking toward him. Mr. Darabont actually said that to me, "try not to blink". So we did the shot and when he yelled "Cut", he jumped up with excitement and I knew he liked what he saw on the screen. That is probably the best highlight of my entire career to date!
There's also a good deal of evidence in the novels.
Walkers' eyes are often described as "sharklike" in the novels, suggesting that they do not blink:
The crowbar impales the cadaver through the roof of its mouth and gets stuck. Bob gapes in horror. Behind the mosaic of fractured glass the skewered head hangs suspended in the wind for a moment, the dull glow behind its sharklike button eyes still animated, the mouth still pulsing around the iron as if trying to eat the crowbar.
—The Walking Dead: The Road to Woodbury
We know that walkers lack other eye-related reflexes, such as saccades (i.e., their eyes don't move in response to stimulus, but rather stare straight ahead).
One female and one male, each one with a gaping divot for a mouth, their bloodless lips oozing black bile, their shoe-button eyes fixed and glazed.
—The Walking Dead: The Road to Woodbury
Some walkers have lost their eyelids, which provides further evidence to suggest blinking is unnecessary for walkers:
Jagged, misshapen knots of bone protrude from the corners of its limbs, giving it a ghastly puppetlike appearance. Perhaps once a child or a dwarf, the miniature humanoid stares through luminous, lidless eyes as it gnaws at Hap’s arthritic joint, sucking the blood and marrow with the fervor of a starving castaway suckling the last drops of moisture from a coconut husk.
—The Walking Dead: Descent