Best method/techniques for creating mouths so charactors can talk?

I have made a few models so far with lips but only problem they cant actually talk only smile or frown. I would like to use these charactors and not have to rebuild them. Is there anything I can do to create a mouth so they can talk and i could do lyps synch. What is better for this shape keys or bendy bones. Any good tutorial on creating the mouth from the begging on the modeling process so I avoid this problem in the future?

I am using face rig with bendy bones to deform mesh nicely. You can pose face bones to represent this or that phoneme or expression and easily apply these poses, also you can tweak any bone to get result you want. With shapekeys it’s impossibly. Also face rig can be easily shared between different characters with different topology. Shapekey can’t do that. Tho you can combine face rig with shapekeys to make small deformations like wrinkles with shapekeys if your mesh is dense enough. This tutorial is good point to begin with:

This one is good also - using drivers to deform lip cornes scaling control bones:

I personally like shapekeys, as they are a lot easier to set up - and if you want a particular expression you can just add another shapekey.

Yes you talked about animating facial expression. But how do I model my charactors from the beginning to include a mouth? Like I said I made a charactors but it cant open its mouth only smile or frown. What should I do Any video that cover how to model a charactors to include being abple to open the mouth?

What about modeling? I created a chara tor but I totally forgot that it needed teeth and a tounge for when it talks. Any videos that cover proper set up and modeling from the beggining?