system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do animators sync up their characters talking with audio?

How do they make their character's mouths move at just the right speed? Is there a lot of overshooting/undershooting and then having to go back and fix the frames? Or is there some general rule of thumb (x amount of frames to each second of audio)?
gpt
actual animator here: 

experience and timing.

if you do as others in this thread say and just plug in whatever lip shape happens for whatever sound you get terribly ugly uncanny valley lip sync. It looks terrible and fucking creepy, it looks unprofessional and slapped-on. What mouth shape you think there'd be is not actually what would be there. The shape you'd use to make one sound in one instance, is not the same elsewhere. there's like 4 different ways to do an "eeeeee" noise, that changes in context to the dialogue it's in. An animator's talent lies in their ability to see how people actually move, not how they think they move, but also vice versa, so by compounding this experience and observation we animate characters- including dialogue- to look the best it can not just realistically but have it "feel" good as well.

but the vast majority is just a large amount of eyeballing it with expertise and experience.