What actors play these characters onscreen?

That’s what this technology will change. If you picture ‘insert script, receive audio,’ then yeah, it’ll suck. But if you see it as a tool - it’s going to let artists create a performance, without a performer. It’s an instrument they can guide and tweak and endlessly fiddle with.

Or more directly, it’ll let someone say a line like they want it to sound, and then transfer that to someone else’s voice. And it doesn’t need to be someone real.