Thanks Robert your thoughts on dialogue align with what I think is important (priority-wise).
I keep thinking that where it comes to mouth shapes I'm mostly insterested in a japanese phonetic approach to shaping (primary of the open/close variety). Namely:
A (Ah - as in 'Ah, so that's how it's gonna be')
I (E - as in the letter E)
U (Ooo - long drawn out)
E (Eh - as in 'Eh? What was that?)
O (Oh - as in 'Oh, Oh, that's gonna leave a mark')
Those five shapes are the primary open/close variations.
The accents then are the consonents
M and N are mmmmostly closed (M usually being the initiator and N being the closer)
I'd say a lot of personality shots can be pushed by moving the jaw slightly to the side (ala a person who speaks largely out of one side or the other of his mouth).
As you well know, the joy of dialogue isn't even in the facial movement but in the accent of the body itself, with hand gestures and poses that convey something other than what is being conveyed in the dialogue.
Too often we see dialogue heavy pieces that explain (or reexplain) what the visuals have already sold to the audience.
Whereas the dialogue would be better served by revealing through words what is in the mind of the character that the visuals don't show.
For instance, a disinterested character that is pretending to pay attention.
Or perhaps better yet... as a foreshadowing of something that will be returned to later as we learn more about the character.
As you say though, I think the simple open/close of the mouth can serve the job best in many cases.
Then it's mostly a matter of tying the facial poses into the intended emotion of the character.
This is even further demonstrated by the many shots that have a character talking while the camera is fully focused on the reaction of another person who is (suppose to be) hearing the dialogue.
In those instances not only does the sound hit its cue but we get to see the reaction of what another character perceives concerning the dialogue.
It is through that secondary character that we (in the audience) are able to assimilate the true intent of the purpose/existence of the dialogue.
Time after time after time we see that most dialogue is unnecessary and yet scripts are still packed full of the stuff.
I see two primary purposes to dialogue; the first is to intentionally lead someone astray (regarding the plot); i.e. to believe something other than that which needs to be reserved or withheld until later in the plot. The second is to clarify and or resolve such ambiguity so that misunderstandings do not happen. Of course, some dialogue intentionally leads us to believe that we, along with the characters, don't have a clue what has happened (i.e. dangling plots).
Added: I do think that something most cg characters lack is ability to really exaggerate mouth shapes. We really need to be able to show those toothy grins, puckered lips and such to gain enough exaggeration to push those personalities. Of course this is mostly for characters whose performance will be highly energetic. One who isn't energetic might have very little mouth movement consisting of mumbling to themselves.