I wonder how being an actor in an interactive holodrama would work? The program would need an almost infinite number of actions from the character in order to respond to whatever the person playing the program does.
Not really. Just enough of a range of samples to extrapolate from. We're all capable of only a finite number of facial expressions, body movements, vocal intonations, and so on, so a computer would just need a reasonable-sized catalog of possibilities to build performances out of. Remember how in
Mission: Impossible III, Ethan Hunt made the bad guy read a sample text from a card so they could program the voice synthesizer to mimic his voice? The idea was that the text on the card included all the basic phonemes needed to construct every possible word. You needed that real-life baseline data as the raw material that the computer would then piece together to create a convincing synthetic voice. This would be the same principle, only more complex.
And of course, a lot of the content of any holodeck game, like any MMORPG today, would be preprogrammed dialogue and interactions, so much of it could be directly recorded from live performances. The player's interaction with the characters might just cause the computer to select one of several possible preprogrammed reaction sequences, and it would only have to synthesize a reaction from scratch if the stimulus didn't lead to one of those possibilities. We saw something like this in Picard's Dixon Hill games, where the characters would pretty much just ignore any player statements or actions that didn't fit their preprogrammed scripts, or would just go "Huh?" and then resume doing what they were programmed to do -- again, much like computer game characters in real life.
They're already doing CGI face replacements in movies like with Tarkin in Rogue One, but there's always an actor who provides the basis for the performance, in the case of Tarkin it was
Guy Henry. Then of course there's Andy Serkis, who at this point has probably provided more motion capture performances than any other actor.
Although in both cases, the performance capture alone is not enough, and it takes the work of multiple skilled animators to add convincing texture and life to the performance. One example I've heard them use in talking about Henry as Tarkin was that he didn't move his mouth quite the same way as Peter Cushing when pronouncing a certain sound, so the animators had to tweak his performance-captured mouth movements to look more like Cushing's -- fine details like that, stuff that non-animators would never think of.
Serkis is the best-known performance-capture actor, but probably not the most prolific; that's probably someone who does background characters, who might "play" numerous different supporting roles in a single movie. Let's say Serkis has probably played more
leatured performance-capture roles than anyone else.