How far away are we from malicious "putting words in someone's mouth" a la "look - a video proof of my sociopolitical nemesis saying something utterly reprehensible!"?

Not a film, but I'd say we're pretty much there in video games with roles like the one Ellen Page had in Beyond: Two Souls. Note that she even explored legal action against the game company because of an unauthorized scene.

I would be more interested to see where we get with synthesis actors. I could see them creating a single recurring screen actor from 3 or more human beings. One or more for appearance. One for particularly good facial expressions. Another for voice. Maybe another for movement.