Short answer then? No, you can't, from scratch create and animate a CGI model that's going to be THAT believable. Its too complicated to be practical at this point in time.

Still image? Absolutely, there are lots of artists out there that could put a CGI model into a still image, and you'd never be able to tell the difference. Animated tho? Nope.


Longer answer? There's a lot of nuances to what you're asking, you seem like you have something specific in mind, but you're a bit vague so I'm not sure what direction to go, perhaps if you laid it out/asked directly, someone could give a clearer, more direct answer, but in any case, here's some thoughts ...

Can a program be written to automatically make the little movements? No, not really. There are just too many, if we can't manually animate them, we can't realistically write a program to account for each and every one, and every permutation thereof, its not necessarily a "math" type function, but that's where machine learning/AI would come in. You might be able to train an AI [eventually] to account for all that stuff.

When I say "little movements" I'm talking things far more granular than the movements you'd find in an action like "walking" [not that the physics sims in clothing wrinkles/etc aren't insane in their own right, and still bleeding edge], but I don't know that you'd necessarily want to "program" a model to walk anyway, you'd probably just want to use motion capture for something like that, [assuming you didn't just do a head replacement over an actor's body, you've been vague, I don't know if that's within the realm of what you're asking]. Motion capture, not exactly "animated" or necessarily "from scratch", if that distinction matters for what you're asking, but motion capture would get you a long way in the right direction. Actions like walking [at a distance] have been done that fairly convincing for years now with motion capture. Big actions/movements like that, aren't really the problem. Hands will still be tricky, but I think we're probably at a point with enough physics sims and a complicated enough rig, you could sell those. Its the face tho, its always going to come down the face.

A full facial motion capture setup, would be a good place to start, the more tracking, the better, but that comes with caveats. You'd need an actor for the mocap performance, that had a  similar facial structure to your 3D model, the more differences between the two, the more obvious problems will become. Full cheeks will move different than gaunt cheeks, smile lines, different creasing in the corners of the mouth, wrinkles/folds around the eyes, etc etc, the more of all those things in common between your mocap actor, and your 3D model, the better. Then you'd need an elite animator to clean it all up.

There are just so many tiny details. The ears shifting when the jaw flexes, the nose stretching when the mouth moves, hairs shifting when the brow furrows, those are "obvious" things to take into account. The cornea of your eye sticks out of the "round" of the eyeball, which causes subtle shifts/stretching in the eyelids as the eyes move around, which cause individual eyelashes to change directions slightly. The way the pores in your skin expand/contract/stretch with the skin. That's moderately obvious stuff. Then there are far less obvious things; take your hands, and place them on the back of your head, a little above ear level, then raise and lower your eye brows, you should feel that action on the back of the head which is causes the hair to shift. Is it something you'd ever consciously observe happening on someone else? No, and if just that one thing was missing it probably wouldn't be an issue, but when you've got that, and hundreds of other "micro" details compounding, it starts to slip into the uncanny valley.

Can a program be written to make all those movements? Maybe? A number of the foundations are out there already, kinda, if you really pushed them. For example maybe set up a few dozen different physics sims to capture more of the way a face realistically stretches. I can't remember now, its been ages, but there were some studios/artists working with creating all the individual muscles of the face rigged with physics, over a model of the skull, and then a physics sim for skin over that, all being animated, giving varying degrees of success. Stack multiple hair simulations to get something more natural than what you commonly see. Get an animator, that really, REALLY knows facial animation, and put in probably hundreds of hours per second of animation. Then maybe you could expect results that would fool everyone.

Thousands of computer hours on physics simulations, and hundreds of man hours animating, plus mocap and everything else for every few seconds you needed animated? Its just not practical.

Without really knowing what you're wanting, how granular you would need to get would depend on what the shot[s] required. You could probably-maybe, do it for a mid-long range shot, full body/full frame at 4k resolution. I could see that being feasible. For a mid-shot, waist up at 4k, ehh, maybe, if you really really got down to it that would be possible. For a tight shot tho, head and shoulders at 4k, nah, we're just not there yet.

Keep in mind, this is all from the standpoint of, "what is practical"? If you throw enough time/resources at the project, then sure maybe, but there's just no scenario where anyone is throwing 3 years of work at a 5 second animation. If you did that starting now, with the current technology, by the time you finished, tech will have come along that can do a better job faster either/or you get caught in a loop where you're constantly iteratively incorporating all the new tech that comes along, continually refining, endlessly, so practicality forces us to settle for "good enough for now" which, just isn't quite there yet.


But, so like, you say maybe it could be done if you went ham on it? How would do it? Like, life-or-death, "deliver a convincing sequence or be dropped into the depths of the ocean wearing only cinder blocks shoes"? I suppose I'd try some sort of hybrid approach. Motion capture as a foundation, layer it with heaps of physics/hair simulations to fill in the gaps the mocap didn't pick up, refine that frame by frame and when its almost, ALMOST perfect ... deep fake it? Instead of putting one persons face on another, I'd be using the nearly perfect CGI to train the AI and put that over the actor I was replacing. Would it work? Maybe? I think there might still be some tells, especially if viewed hyper-critically, but on the flip side I'd likely buy myself a year or two longer to live before I had to put on the cinder block shoes, cause it would take forever to do all that.

Maybe if the deep fake using the CGI model worked well enough [I'm spit-balling with that process, I have no idea if you could actually train an AI using a CGI model as the basis for a deep fake like that, I haven't trained an AI before], you'd get to a tipping point where it does start to become practical, where your initial few thousand hours invested gives you the ability to turn out much longer sequences using a deep fake method.

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