kaogei 顔芸

In Japanese, kaogei 顔芸, "face performance," refers to comedic performance (in real life) using the face, specially making weird facial expressions.

From that, in anime and manga, kaogei can refer to basically any facial expression used to communicate a characters' emotions, but specially the intense, ridiculous, or over the top ones, like those often used when a character is shocked or enraged.

Examples of 顔芸, exaggerated anime facial expressions.

Anime: Shinchou Yuusha 慎重勇者 (Various Episodes)

Definition

The term kaogei can refer to a bunch of things in anime. Like:

Weird faces.
Funny faces.
Over the top faces.
Distorted faces.
Reaction faces.
Faces that show intense emotion.
Faces that show an intense change in emotion.

In other words, practically anything related to facial expressions IN ANIME can count as kaogei.

I mean, given the origin of the term, you would think kaogei should be limited to "weird faces," hengao 変顔, and maybe originally it was limited to that, but nowadays it can be used toward pretty much any noteworthy facial expression.

To begin with, "weird" is subjective. A lot of reaction faces look weird, distorted, deformed artwork-wise, but are they all kaogei? Where do normal facial expressions end and kaogei begins? It's impossible to draw the line, therefore anything can be called kaogei.

Character: Ristarte リスタルテ
Anime: Shinchou Yuusha 慎重勇者 (All Episodes)

Given this, rather than whether something counts as kaogei or not, it's more about whether the expression is worth noting or not. It's just like how all drawings are "art," but nobody ever says "this anime has ART." They say the anime has GREAT art, or SHITTY art, but not just art.

Same thing with kaogei. When the facial expressions in an anime are worth noting, kaogei is the term.

Note: if a face looks weird because it's not drawn with the same quality as the rest of the series, that's called sakuga houkai 作画崩壊, "artwork collapse," not kaogei. The term kaogei refers only to faces that were intended to look the way they do.

Bad quality anime face and its improved version.

Anime: Kamichama Karin かみちゃまかりん (Episode 4)

This infamously bad-quality anime face was broadcast on TV (top), but redrawn in the DVD release (bottom), which means it wasn't supposed to look that way originally, it just ended up that way due to production issues.

Also note that kaogei doesn't refer to any specific facial expression.

Gesugao of and Saotome Mary and Jabami Yumeko.

Anime: Kakegurui 賭ケグルイ (Episode 1)

These are examples of gesugao ゲス顔, a sort of facial expression.
Since gesugao is an unusual face, it also counts as kaogei.

Most of the time, kaogei refers to the weird faces that a character makes reacting to something. Like, someone does something stupid, another character is shown with a face of shock, surprise, dumbfoundedness, anger, and so on.

Since these weird reaction faces can be funny, and storyboard-wise what's shown to the viewer is: first, the stupid scene, immediately after, the reaction, the kaogei can be hilarious on their own, just because of the sudden ridiculous face you find yourself staring at.

Anime: Dr. Stone, ドクターストーン (Episode 3)

However, not all kaogei are reactions to things other characters do.

Sometimes a character is thinking something, like scheming something, or thinking something perverted, and they make weird faces, too. And that counts kaogei, too, because why not?

Anime: Asobi Asobase あそびあそばせ (Episode 1)

In less realistic anime, comedies, the art style can become extremely simplified and distorted to emphasize the emotions. Those distortions count as kaogei.

deforume
デフォルメ
Deformation. Distortion. When a character is drawn simplified. For example, konsento-me コンセント目. (from French "déformer.")

Character: Kotobuki Tsumugi 琴吹紬
Anime: K-On!!, Keion!! けいおん！！ (Season 2) (Episode 14)

A girl drawn blobby, not to be confused with moeblob, which means something else.

On the other hand, sometimes the art style suddenly becomes more realistic than normal when the point is emphasizing the gravity of the situation, or when the character is being very creepy. That, too, counts as kaogei.

gekiga
劇画
Dramatic picture. Referring to manga drawn in a more serious tone and art style, featuring more serious stories. Can also refer to kaogei when the characters' faces look like they come from such a serious manga.

In more realistic, serious anime, there's no kaogei, in the sense of distorted faces, because the faces in realistic anime have to look realistic, so you won't see a jaw literally dropping on the floor and ignoring all laws of physics on its way downward.

However, sometimes people use the term kaogei to refer to faces showing intense emotions, whether realistic or not, because, again, why not? Indeed, the term can refer even to rather intense expressions that aren't even distorted or anything like that.

Just... expressions in the general. Honestly.

There's even a meme involving the kaogei (i.e. total lack of poker face) of Sonoda Umi 園田海未 from Love Live!^{[海未ちゃん顔芸シリーズ - dic.pixiv.net, accessed 2019-12-31]}

【ラブライブパロ】ロゼッタ様がババ抜きに挑戦したようです

ゲームに負けたキタキツネ

どちらに投票しようかな?

Romance manga that use the classic shoujo art style, making characters pretty and beautiful, tend to avoid kaogei. That's because simplified faces look cute, but in the childish sense, it doesn't look beautiful, and weird faces don't look beautiful, either, so it doesn't benefit the manga anyway.

Some people don't like kaogei, they don't like over the top faces showing up everywhere. Or they think kaogei sucks if it's done too much. Because it's kind of the the anime face equivalent of laugh tracks. Or maybe they hate kaogei that makes faces look uglier than normal, like in GTO.

Other people do like kaogei, which naturally means that there recommendations for anime with kaogei exist. For example, the following are some anime in anikore's 顔芸 tag, accessed 2019-12-31: