I mean, that works for your own workflow - awesome. That's great when you are an indie creator and you're doing all the acting yourself. 

In my case, I have professional actors who are delivering a full performance and I am coming along afterward and cleaning up their lip synch basically, trying to preserve the rest of their performance as accurately as possible, not the other way around.

The technique described with the motion puppet gets me part of the way there but doesn't address my question in its entirety - that's a great technique for completely removing all mouth movements off your mocap so you can entirely replace with AccuLips.

That's not what I'm looking to do - I'm looking to BLEND the acculips and the mouth part of the facial performance, while KEEPING the rest of the facial performance at full strength.

So far I haven't seen a way to control the amount of the blend on masked-out parts. I know there's no feature to do this directly but I was hoping some clever user had come up with a way.

Try this manual page for starters:
https://manual.reallusion.com/iClone-8/Content/ENU/8.0/50-Animation/Facial-Animation/Removing-Facial-Expressions.htm

Maybe my brain isn't working well, but my workflow is to always create the visemes with Acculips and then layer the ENTIRE facial performance on top of that (with Live Face).  Doing it that way gives you exactly what it seems you are looking to do, namely - get the accuracy of Acculips with the facial performances of an actor.

I don't even do much on top of that other than I do not speak the lines - which is to say I will smile or frown (or whatever) and sometimes I even open my mouth a bit more of less but putting that layer on top of Acculips gives, to me, the perfect blend.  No messing around with anything.

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Alienware Aurora R16, Win 11, i9-149000KF, 3.20GHz CPU, 64GB RAM, RTX 4090 (24GB), Samsung 870 Pro 8TB, Gen3 MVNe M-2 SSD, 4TBx2, 39" Alienware Widescreen Monitor
Mike "ex-genius" Kelley

I have lots of already recorded body and face mocap. The face mocap is animating CC4 Kevin's full face, including mouth.

I want to use 100% of the expressiveness of the facial performance BUT I want to blend 50% of the lips performance with 50% of the AccuLips viseme performance. If I just play with the Viseme / Expression strength sliders, they affect the WHOLE facial performance, resulting in me losing the eyes and eyebrow expressiveness. 

I don't know how to either
1) EDIT the curves of the facial mocap so I can smooth / reduce keyframes on the LIPS/MOUTH ONLY
2) Mask out the Expressiveness setting so that I can have 100% expressiveness of the eyebrows but 50% on the lips.

I know I can replace/rerecord a facial mocap performance, but I used real actors and I need to preserve their performances.

Any thoughts?