Lip sync can make or break your character animation - done well it can take your character's performance to another level - done badly it can turn your shot into a train wreck. This post is for the animator who submitted the offending shot, and the producer or director who can see that it's not working but aren't sure what's wrong with it.
1. The dialog seems too early or late
I've seen it happen more than once where the dialog has accidentally been nudged along the timeline a few frames - so my first take on this would be to slip the dialog track back and forth on the timeline and see if it falls back into sync.
Offsetting the lip sync can actually help - sometimes the mouth shapes may be accurately timed to the dialog sounds, but it still doesn't seem to be in sync when played back. Try offsetting the mouth key poses a frame or two earlier than the corresponding sound - this can improve the readability of lip sync. The exception is the closed mouth sounds B, M and P, which should fall right on the closed mouth pose.
2. The mouth shapes are too big
If the character's mouth moves through BIG shapes for open mouth sounds like A, E and O - this can look pretty grotesque. An animator tackling lip sync for the first time might have referenced a mouth shape chart like Preston Blair's famous 'DIALOGUE' page and created key mouth poses that are oversized. Lip sync mouth shape charts often show exaggerated poses - to make the point, rather than to be followed verbatim. Scale down the size of the extremes of the mouth shape and try it again.
BTW - that single page was the only published document on lip-sync pre-internet days, and a generation of animators owe a debt of gratitude to Preston Blair (RIP) for getting them out of trouble.
3. Too many mouth shapes
If the mouth is moving through too many shapes in the dialog it will looks jittery and unnatural. It's a common mistake to try to hit every single phoneme with a key mouth pose. Identify the important shapes that define the word, and skip over the lesser ones.
Generally you should try to create lip sync with as few mouths shapes as you need to make it convincing - real speech is like that - the mouth doesn't move all that much. An extreme is anime - whose limited animation style typically achieves lip sync with two mouth shapes - open and shut (millions of anime fans never complained about this).
The best way to check what's really going on with speech is to rehearse it in a mirror - and all good animators have one on their desk - right?
4. The wrong mouth shapes
If the mouth is open when it should be closed or wide when it should be narrow, it should be pretty clear that the wrong mouth pose is being used. I know it sounds obvious, but I've seen it many times and unless it was somehow accidental - there's really no excuse for it. As a supervising director once told me - animators who do this should be publicly flogged. That might be a little harsh - but you get my drift.
5. No follow through
It's not a good look when a mouth slams shut - back to the default closed position at the end of a word. Unless the word ends on a closed mouth shape, the last shape of the spoken word should hold for a while.
The final word is that there are no hard and fast rules for lip sync. What works for one character design or animation style, won't necessarily work for another. It may seem bewildering at first, but once you nail it - it's magic. And lip sync is just a subset of facial animation - which is another blog post.
Sherazade The Untold Stories (2017) is an animated TV series initially developed by Hahn Film. Images are from season 1 episode 10 and episode 17, directed by Steve Bristow.
Here's a link to the storm sequence from episode 17
Got any more lips sync tips and tricks? Post 'em as a comment.