Some very famous and respected actors have a deficit that is not so much they can't act, but that - for whatever reason - they just don't act.
The much awarded Eddie Redmayne is one example: he just doesn't act.
I'm not sure how I would describe what he is doing, but it isn't acting. Just look for yourself - setting aside that you have been told he is a great actor - and you will see; especially if you compare what he is doing with somebody in the same movie who is a good actor.
This effect can be simulated by bad direction or poor script working on a good actor. For instance, Nicholas Cage is an excellent actor, but has been in a a lot of badly-scripted and/or poorly-directed movies, so you might get the impression he is a bad actor.
On the other hand, to be accurate; at his worst, Nick Cage seems like a bad actor - but what he is doing is, nonetheless, still "acting". My best guess is that Eddie Redmayne can't act, since he apparently never does, and always seems exactly the same.
I have reached the same conclusion about Millie Bobby Brown. She is neither a good nor a bad actor: she just doesn't act at all. Which is probably why everything she is involved in, that I have seen, was embarrassingly awkward and the ensemble just didn't work.
Both ER and MBB have a an inhuman, android, replicant quality about them; a deficiency of empathic connection; which reveals itself in blank eyes...
It must be a literal kind of nightmare for an actor to be compelled to participate in a scene with one of these not-actors. They function like anti-drama vampires. It's almost a "super villain" ability - the ability to negate the acting-powers of any actor within their force field radius.
Don't-act-ors destroy dramatic coherence and make everybody else look bad, by themselves being sub-bad to the point of making a powerful impression on the casual viewer.
They exist like blank-eyed rocks, while all around collapses into frustrated impotence.
And that's apparently what gets you the big awards nowadays!
