I think he is one of the most believable of actors. In "The Third Man" and "Sons and Lovers," the context is as important as his character. This might mean that he doesn't come through as flamboyantly as some, but in staying with the overall flow of the film he inhabits, he can still be individual and, also, realistic. For me, this is the best kind of acting.
A very good actor, but so understated in many of his parts that he seems hopelessly out-of-date compared to the showboating, sub-Brando wannabe "serious" actors that we have now.