How can I create a custom viseme file that can be used with multiple characters.  Not just expressions, but lip talking scripts

Try this: create the viseme animation as normally would, do a Collect Clip, and then save as MotionPlus file. 

The MotionPlus file will contain animations, expressions, and visemes, including the actual speech file.

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The only other way I know of is to use CrazyTalk, which lets you create an actual rlTalk file. This can then be loaded in iClone as a Voice Script.

Details here: http://manual.reallusion.com/iClone_6/ENU/Pro_6.0/08_Animation/Facial_Animation/Creating_RLTalk_and_Voice_Script.htm.

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good idea, thank you