 Preparing the model This way of doing facial animation should be considered when starting and through the process of creating the character.
Once we have the mesh of our character mesh ready, we can start getting some references from other games and other pieces of media to choose the extreme poses they'll be able to perform with facial animation. I'm going to choose one of my favourite 3d animated movies as an example: Wreck it Ralph. Let's use Vanellope since the proportions for the example is closer to hers more than Ralph.  So now we have some references, let's plan in how many parts we need to divide the whole puzzle to make it work... We need to separate:
 VERY IMPORTANT: after duplicating and separating the meshes from the original one, you can see the cuts where the normals broke. We can use Maya's Transter Attributes to copy the normals from the base mesh. 
See? Much better! None of the cuts are visible :)  Texturing the model Let's start texturing the eyes. The principle which all of this system is based on is an offset the UVs of the mesh or the material. Once we have the eyes in place let's proceed with the eyelids and make them blink!  As you can see, we need to leave some extra space on the sides of the eye so we only get one iris for each eye, even if we push the UVs to the limits. When painting the eye, I use the Base Iris we already have to draw the eye itself, but the point will be to leave only the eyelids and the red part as opaque, and leave as transparent the center so the eyeballs can offset under this new layer on top. Let's draw the rest of the frames and then we'll organize them in a texture atlas!  This is the result of the 4 frame animation.  Let's now organize them so we can have more variations with other expressions in the future! We can use a 4x4 grid, giving us 4 animations of 4 frames each, 2 animations of 8 frames, or however you like to organize it! Thanks to Photoshop guides we can organize them in a regular way, with 1/4 of the total width of the texture.  The red parts are going to be painted white so we can change the skin tone inside Unity, and the insides of the eyes are going to be removed so they're transparent, looking like this when exporting the texture:  The checker pattern is the transparency bit. Let's import our character into Unity now and try how our system works. Assign all the materials to the correct mesh. Make sure the Eyelids' Material is set on "Fade" so it lets the underneath colours go through.  Animating the model The next part is where different workflows can be used to make the eyes, eyelids and facial parts animate we setup before. The optimal way to do this would be to setup a script in Unity that receives the offset of different facial parts with a setup of bones that control those offset from Maya, so the Animator in Maya can see what they're doing. It's a more complex topic and I hope I can make a blog post in the future about that. We can also setup an animator that controls each part separately to modify the offset of the materials (this is the one we're using in this example since it's a very simple workflow, and can be compatible with Mixamo animations), but it's not really optimized.  We have to be mindful of the type of curves we're creating for a sprite sheet kind of texture (eyelids in this case), since if it was linear it would look super strange panning from one frame to the next. We need it to snap from frame to frame, hence we'll be using a "constant" type of curve.  Here you can see the difference between using "constant" and using "linear"
  Conclusion There are many workflows to achieve the same result, and in this case we might get carried away about what should we use, either blend shapes (morph targets) or bones for a facial rig, but there are always more options available to you, and the games from Nintendo make a really good use of "now forgotten" techniques that were more common in the past.
Hope you learned something new today, and that it was useful. See you around! :)
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