You might not know it from reading this blog, but I was a working animator for decades and am teaching the nuts and bolts of animation right now.
This is something that I figured out while trying to explain lip synch. I don't believe that it's in any of the instruction books. If it is, I wish somebody would let me know, as I'd like to know if I figured this out on my own or unconsciously stole it from somewhere.
Since I'm now making PowerPoint presentations available to my students, I suspect that the files will circulate far and wide as animation notes are likely to do. So I'm staking my claim to this here, assuming that I haven't ripped it off myself.
We exhale when we talk. Generally, vowel sounds are an unrestricted flow of air from our mouths and consonants are a restricted flow of air. The shape of our lips is crucial to getting a vowel to sound right, but it's not nearly as crucial when sounding out a consonant.
If you say "steam room," you'll note that both words end in an 'm' sound, but that your lips are in different positions for each 'm.' You'll also note that your lips don't just move up and down, they also move away and towards the centerline of your face. That's one tip for making your mouth action feel less mechanical.
However, the thing I figured out works out to be a general rule. Say "raid, rod, rude." All three words end with a 'd' sound, but your mouth is in a different position each time. It turns out that the mouth shape of a consonant is determined by the mouth shape of the preceding vowel.