I have seen the tutorial where RL suggests removing the jaw bone to close the NPCs mouth in Unity when it is animated, great for nonspeaking NPCs.  But this fix only works for NPCs that don't talk (aka don't have viseme tracks).  So my NPC moves and talks in the Unity scene.  All works great other than the NPC has a mouth hanging open during times when they are not speaking.   All my iClone animated NPCs look like this when they are not talking.  I'm guessing there is a setting or two that I don't know about or some other thing.  Is there an iClone trick or setting or a Unity trick I should be using when I create my animations for this that I don't know about?  
Thanks!

Not sure I ever tried recording the idle within iClone with a viseme track with their mouth closed.  I'm going to try that but if anyone has a suggestion or trick for this, love to hear what you suggest!

It's the infamous Unity open mouth problem.

Basically, the Unity devs decided that the default state of the jaw should be open. So to close the mouth, the animation track must explicitly close the jaw.

But: Most animations from the Unity asset store or Mixamo or most anywhere else, don't usually have Jaw bones, so the animation clips generated don't write the "Jaw Closed" track, so the mouth stays in it's default position, which is open.

The solution is to edit these problem animation clips and set the "Jaw Closed" to 1.0 at both ends.

A feature that has been built into my Untiy Auto-setup to correct troublesome animation clips, you might find useful (Starting at 1:20):

thanks a bunch, I'll try adding an event at the beginning and end of the idle animation I guess - to set the jaw to 1.  Nice added tool.
 I'm forced to use Unity 2018 for this project as I'm creating on top of an API on top of Unity 2018 :-( boohoo.
  Is your add-on tool only for 2021?  It looks great.

Off to try this, thank you.

It's for 2020.3 and up.

But there is an editor script in there: QuickAnimProcess.cs, which will work standalone and adds a right click menu that will clean up and save fixed copies of all the open jaw animation clips in a model file.

Interesting that the default state of the jaw in Unity is open. Like "Duh"? 

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It seems ridiculous and it confuses the hell out of everyone.

But there is a certain logic to it. Unity's humanoid animation system is based on muscles, and it's more like a muscle simulation, which is how it retargets between different animations and avatars so easily.

So zero values are the muscles at rest (open), and larger values are the muscles contracted (closed).

But as the Unity devs never really bothered to explain any of this (or if they did it's been lost to time), the side effect is that everyone's characters stand around looking like gaping mouthed fish and wondering how to fix it.

They did briefly wonder, back in 2014, if they should do something about it (https://blog.unity.com/technology/mecanim-and-default-values), but must of thought better of it.