This is another artistic convention seen in anime and manga, and most commonly in comedy material.
Even when a character is facing away from the viewer and only a narrow part of the side of the face is visible, facial expressions are still apparent, especially a sneaky smile that crawls up the cheek and almost reaches the ear. Real flesh-and-blood humans are anatomically incapable of this, but drawn characters suffer no such limitations.
Can be used to hide a Psychotic Smirk.
- Early in Inuyasha, Sesshomaru smiles like this... and the smile keeps going, becoming a fanged muzzle as Sesshomaru takes on his true dog-demon form.
- Haruko in FLCL does this a lot.
- Kamon as well.
- The OVA version of Hellsing does this a lot with the "face splitting smiles" of the original manga.
- Many characters in Crayon Shin-chan do this, though Shin and Hima do it the most. For those two the smile isn't just visible from the side but directly behind them.
- Nausicaä of the Valley of the Wind. Kurotowa seems to have one perpetually plastered on his face. Ironically, Chris Sarandon seems to share that trait.
- Lust does this early on in Fullmetal Alchemist. Combined with Psychotic Smirk.
- Plenty of characters in Soul Eater get this, particularly Soul himself when the little demon gets in on the action.
- Both Kamina and Simon both do this the course of Tengen Toppa Gurren Lagann, especially when they each both first get in their own mecha.
- Tora's default expression in Ushio and Tora, both happy and angry.
- The semi-animated opening credits to the 1960s TV series Batman.
- Toph in Avatar: The Last Airbender uses this whenever she's about to do something impressive.
- Girl Genius does this all the time, especially with the Jägermonsters.