The crew's portraits at the bottom right of the flight screen, drawn as often as the game itself. The game redraws them only seven to ten times a second, which is why the faces jerk. Nothing else about the picture is changed.

For Kerbal Space Program 1.12.x. Needs Keystone, a separate download (on SpaceDock): this mod does not load without it.
Copy the SmoothPortraits folder from the download into your KSP's GameData, beside Keystone. It is on as
installed.
In the Keystone window (the keystone button on the game's toolbar, or Alt-K; Option-K on a Mac): the most pictures a second (60 as installed), the most of each frame's time the portraits may take (3 %), and whether kerbals outside the ship are included.
Windows 11 and Mac: tried. Linux: never run (it is plain C#, with nothing of one system's in it).
More: the long description (everything in full, and what was tried on each system) and the rest of the wiki.
CC BY 4.0: use it, change it and pass it on as you like, as long as you credit Ishiaki and link back here.
Raw stats are from the beginning of time until now. Each follower and download entry represents one hour of data. Uneventful hours are omitted.