I was watching Tom Hiddleston playing Loki in a not-particularly-good movie called Thor: The Dark World, and this confirmed to me that he really is an actor of genius; by which I mean something quite specific. He is incalculable in his acting, and produces flashes of the uncanny - surprises which give an unpredicted jolt of rightness.
It is interesting, and unusual, that Hiddleston is presumably extremely intelligent (he apparently achieved a 'double first' degree in Classics at Cambridge) - and highly intelligent actors (a somewhat rare breed) often lack this special instinctive quality - which is not something than can be achieved deliberately, or by planning.
Once I have seen this from an actor; then even when I see it only in one performance, my genius rating of them is permanent; no matter what else they do.
(Or fail to do - after all any actor depends on the script, director, editor and many others - any of whom can sabotage his performance. This seemed to me to happen to Hiddleston when he played Henry V in the recent BBC Shakespeare series, where the direction was appalling.)
Probably only a minority of the 'great' and successful actors have this special touch of genius (although of course such a negative judgment is contingent - it may simply be that one has not observed the performance when this happened). But I have certainly seen it is the likes of Robert De Niro, John Hurt and Alan Rickman; as well as lesser known actors such as Kenneth Cranham, Bob Peck, Alan David, Fiona Shaw, Laurence Fox (off the top of my head).
Genius has nothing much to do with good looks (although Hiddleston happens to be good looking), or voice (the actors with the nicest voices are - on the whole - among the worst of actors; since they have been chosen for the vocal quality rather than the acting). Anyway, Hiddleston is one of those rare actors that I find riveting - because I never know when he might suddenly produce one of those 'moments' out of nowhere.
And, for me, it is these rare moments that define acting as an art, and indeed make it potentially one of the primary creative arts at the same level as great opera singing from (say) Joan Sutherland or Fritz Wunderlich, or great musicianship from (say) Glenn Gould or Julian Bream.